Jim writes:
I just got new wheels and tires on my 2008 Buick LaCrosse Super, 245/45R19s to replace the stock 18s. The tire dealer put in 40 lbs of air, and when I said “is that a little high?” they pointed to the sidewall and said “look, it can take up to 51 lbs.” I’ve never run more than 36 lbs in my tires, but I’ve never had a profile this low before. Am I just behind the times?
Sajeev answers:
Behind the times? You do own a Buick . . . but there’s an argument for displaced American muscle junkies (of all ages) needing a “Super” for their Yank-Tank fix with a new car warranty. If you believe in the latter, consider yourself off the hook. I agree with your conclusion, for two reasons. That wide of a tire with a 45-series sidewall is far from a rubber band donk-mobile, there’s enough cushion to merit conventional tire pressures.
Reason two: never, ever use the sidewall’s pressure for justification. That number is usually (always?) the maximum pressure allowed by the tire maker, with little relevance to the engineers who designed your car. If my knowledge of the LS4-powered Impala, Grand Prix and LaCrosse holds true, the staggered tire sizes and air pressures of the Pontiac and Buick came from extensive testing on how to put down massive power in a wrong-wheel drive application with less of the Impala’s drama. So don’t discount their calculations: it’s kind of a big deal.
Matter of fact, I suspect the Venn diagrams for tire maker/auto maker tire pressure ratings never overlap. Not that I’m gonna make one and find out, I’m just sayin’.
Sadly, I can’t find information on tire pressure changes for “+1”wheel and tire upgrades. This is what you really need. I’d recommend sticking with 1-2psi more than what the owner’s manual says, or follow GM’s recommendations as-is.
Bonus! A Piston Slap Nugget of Wisdom:
Automotive OCDs unite! If you measure air by “lbs” (pounds) and not “psi” (pounds per square inch), you’ll probably blow up the tire. Yeah, I know that “lbs” is in the American vernacular, but I didn’t ignore wild college parties and NCAA football for Mechanical Engineering homework for no reason. Believe that.
[Send your queries to mehta@ttac.com]
If the point of larger low-profile tires is a larger contact patch, then you would use a lower tire pressure.
Pressure = Force (car weight) / Area (contact patch)
“Yokohama… includes pressure recommendations for stock and plus-sizes, by vehicle, in its annual Fitment Guide, which your tire dealer should have a copy of. Typically, these are 0-3 psi higher for a Plus-one or Plus-two fitment.”
http://www.modified.com/tech/0208scc_tire_pressure_guide/index.html
I generally pump my tires about 6-8 lbs higher than the manufacturer recommends (36-38 instead of 30 on my accord). the rationale: I want precise steering, and I don’t need a cushy ride. Sure, it’s less contact patch, but presumably, higher friction (due to higher weight per sq inch), so I don’t think I’m losing any grip. In fact, may be gaining it since in hard cornering there would be less tire deformation. My understanding is the mfg specs for family sedans are weighted way towards cushy over driving dynamics.
Sajeev,
this is very unclear:
If my knowledge of the LS4-powered Impala, Grand Prix and LaCrosse holds true, the staggered tire sizes and air pressures of the Pontiac and Buick came from extensive testing on how to put down massive power in a wrong-wheel drive application with less of the Impala’s drama.
stock 18s:
235*.5 = 117 mm sidewall
+1 19s:
245*.45 = 110 mm sidewall
The difference is only 7 mm. I think we can safely assume the sidewalls are about the same strength? I say use the pressure listed in the owner’s manual.
Pumping up tires does not increase grip. If it was about higher weight per square inch, why wouldn’t cars use skinny bicycle tires. Higher pressures will result in less friction, and therefore better fuel economy. Lower pressures will give you better grip.
In this situation, I would use what Buick recommends. Remember, the tire is also part of the suspension, and if you make the tire pressure to high, more force will be transferred to other parts, as well as more damage likely to the tire wheel and other suspension components.
The tire mfr’s maximum pressure number is based only on tire safety, with margin for heating, age, and loading. It has nothing to do with the appropriate pressure for the vehicle wearing the tires.
The vehicle tire pressure is based upon distributing its weight evenly across the tread. Too much pressure makes the tire bulge in the center, prematurely wearing the center of the tire, and creating a harsh ride. Too little pressure causes the tire to sag, so that the inner and outer edges wear out early, and handling becomes sloppy, and tire temperatures to increase.
So the proper tire pressure is determined by the car mfr, not the tire mfr, and can be mildly adjusted for comfort, actual loading (like full passengers & luggage), temperature conditions, etc. In any case, I’d say that 40 psi is too high, and you’ll wear out the tire centers too soon.
I test drove a Grand Prix GXP and that car flat out hauled ass and chewed bubblegum. Glad I didn’t buy cause I’d lose my license in a very short period.
GXP came with the staggered tires. Impala/Monte SS did not. With the Super not sure if they are staggered, they also are very rare. I’ve never seen one in person.
Inflating to the max will generally sharpen steering response and enhance fuel economy by some fraction of a mpg. But this comes at the price of diminished grip and a harsher ride, since you’re making the contact patch smaller and the tire harder.
Keep in mind that the tire maker has no idea what kind of car you’re driving when it sets the maximum sidewall pressure. This pressure can make some cars downright skittish. I’d use the vehicle manufacturer’s recommendation as the default choice, and experiment with only modestly higher pressures if you are so inclined. Tire pressure alone cannot turn a Buick into a Corvette, or vice-versa.
sean362880:
I wouldn’t venture to assume that, as sidewall construction can vary greatly from tire to tire in areas such as belt package and bead design, material choice, and material quality.
Also, it would be better if everyone thought of each tire size more as a general “category” than a set of exact measurements: A “235/50” from one manufacturer might be 231mm wide and have an aspect ratio of 48.5%, while a “235/50” from a different manufacturer might have a section width of 237.5mm and 53% aspect.
Bottom line: Always consult each tire’s manufacturer for specific tire dimensions and load capacities at each air pressure interval (some manufacturers supply tables for this).
Hm, so in a similar vein, if I put the exact same size tires by a different manufacturer/series, can I keep the factory stock pressure ?
How about winter vs summer tires?
My 05STi is reccomended 36psi front, 30 back, and I generally keep it at that, but I’m on the original summer tires (I did buy same-size separate winter tires that I keep at 36/30 as well).
I am not an engineer, but I would say that you should follow the recommended tire pressures from the inner door-jam.
I am sure there is a cosmetic reason for upgrading to 19″ wheels on this car because you cannot really be driving it because of it’s stellar handling. Still, increasing the pressure will slightly increase your fuel economy… but again, you surely didn’t buy this car for that reason either.
I would think that the following would most likely be true at increased pressures though:
1- Increased tire wear on the center part of the tire
2- Possible over-heating of the tire (??)
3- Poorer wet weather handling (less contact patch clearing the water
4- Harder ride
5- Increased wear and tear on the front suspension components
6- More ponderous handling than you’ve already got due to the higher unsprung weight of the +1 tire and wheel combo
Jim, the best way to know for sure is to get a tire marking crayon and put some lines across the tread and then go out for a drive. If the line is completely gone then that’s what you want. If the line is still slightly visible on the edges of the tread, reduce air pressure. If it’s visible in the center of the tire, add more air.
David Holzman : Sajeev,this is very unclear:
If my knowledge of the LS4-powered Impala, Grand Prix and LaCrosse holds true, the staggered tire sizes and air pressures of the Pontiac and Buick came from extensive testing on how to put down massive power in a wrong-wheel drive application with less of the Impala’s drama.
I can see the problem. The GXP and Super have staggered tires, the FRONTS are wider than the rear. This supposedly helps the premium torque steering GM sedans maintain grip under load, while cornering. It makes sense.
I also suspect the engineers fiddled with the tire pressures a bit, whether or not they made any change over the V6 models is irrelevant: they probably looked long and hard at tire pressures for this car. ESPECIALLY at this car.
That’s my point.
Paraphrasing several here:
The engineers looked long and hard at the tire pressures before making recommendations…..
Gentlemen –
You are absolutely correct in this statement, but you assume that the priorities given to the engineers match those of the owner/driver.
This may well not be true. The engineers will likely have been given two criteria for a Buick –
1. Improve GM’s CAFE to the absolute maximum.
2. Soft and quiet ride. (Note #1 will trumph #2 in case of any conflict between these priorities).
An additional concern here is dependent on the local highway conditions. That is, the on low profile-tire the tires also act as rim protectors.
45’s are probably tall enough to offer protection in most cases, but not all, if you live in pot-hole country.
So, while I personally use the recommended pressure ranges, I tend towards the high end of the range. I would hope that if you stay within the ranges given that you won’t get much cupping or crowning on your tires.
I would submit that others (like Buick Owners) may want to go towards the low end for a softer ride, quieter ride if their road conditions permit.
I’d also like to mention to regular readers of this column that this is one time where I can’t possibly recommend an LS-X swap.
You’re welcome. :)
Lokkii : This may well not be true. The engineers will likely have been given two criteria for a Buick –
I’d counter that Buick took Pontiac’s GXP tuning lock-stock-barrel in the interest of getting a high performance sedan on the “Super” cheap.
This is General Motors, after all.
A number of the rationalizations given here for lower tire pressure giving greater grip, lower heat, and so-on are (unfortunately) quite wrong, and they are frankly quite alarming to me in their path of reasoning, for reasons that I will detail. Higher tire pressure (within reason) results in reduced tire heat in the vast majority of situations, not the reverse. Destructive levels of heat results in almost all cases from under-inflation, not over-inflation. Manufacturer-recommended tire pressures that were too low for the specified tire and the design of the vehicle is one of the reasons that we had the Ford Explorer rollover debacle, after all. It wasn’t that the tires were actually defective, it was the specified recommendations for their use in application that were defective; this caused excessive rolling deflection and heat accumulation while traveling at speed, which weakened the tires and softened their sidewalls to the point that they failed when emergency cornering maneuvers were suddenly required by the driver.
Modern tires, particularly those that are built in low sidewall profiles for large-diameter wheels, typically use softer sidewall materials than would otherwise be dictated by their engineering requirements in order to ensure compliant ride quality. As such, by their very nature they must require a greater amount of air pressure than a typical passenger tire in order to retain their shape and integrity under heavy cornering forces, especially when the vehicle is one of particularly large mass or high center of gravity… both of which clearly apply to this Buick.
Traditional “standard” air pressures in the United States tend to the 28-30psi range, primarily for pure “comfort” reasons, but also because they’re encouraged by the tire manufacturers to tend to the low side on pressure. They do this because the lower pressure causes higher wear rates and creates market demand for the tire manufacturer’s product, through both the auto manufacturer’s dealer network and the aftermarket tire retail channel.
Different types of tires with different construction characteristics may require different pressures in order to perform properly across the range of their engineered performance envelope. This is the reason that two different types of tires (even in the same size on the same vehicle) will require slightly different pressures to perform similarly, and two similar tires in different sizes (such as 18″ vs 19″ as mentioned here) will also require slightly different pressures to generate equivalent results on the vehicle.
I will say this unequivocally though: 28psi is too low for proper lateral grip and steering performance in over 90% of the tires sold in the United States. A tire that rolls over onto the sidewall area (off of the tread patch) during emergency maneuvers or abrupt cornering is fundamentally under-inflated. The only tires that I am aware of that do perform properly throughout their performance envelope at ~28psi are extremely expensive top tier luxury tires with advanced construction, or motorsport-oriented sport tires with extremely stiff sidewalls.
The pressure that is used in a particular tire on a particular vehicle should be dictated by three things. First, their real-world grip on the road under heavy braking and acceleration while turning. Second, their response and feedback to steering input, both at low speed (city), high speed (highway) and twisting secondary roads (the most difficult test). And third, the pattern and rate of wear across the tread is very significant, and should generally be even from edge to edge if alignment factors do not adversely affect it.
If the tire can be used in any common road conditions, at any reasonable speed, with any level of steering and cornering forces available to the vehicle, and still wears evenly across the full tread width while providing high levels of grip and response, then the tire pressure is correct.
To make a long story short about best pressures for the majority of passenger tires sold in the US: after testing to find the right balance of performance and wear, pressure in the high 30’s is most common, high 20’s is surprisingly uncommon, and some softer tires will actually perform best at over 40 psi. On many vehicles, staggered pressures of 2-3psi front-to-rear may also be appropriate depending on weight distribution and other factors. Beyond that, the only effective solution is to experiment (safely) on your own and find your own best solution for your vehicle, tire type, and pattern of driving.
Good luck!
It’s kind of sad that the tire dealer (of all people) doesn’t know the difference between the number on the sidewall and the one written in the owner’s manual (or door jamb).
Sajeev,
With regard to your comment that sidewall pressures “are usually (always?) the maximum pressure allowed by the tire maker, with little relevance to the engineers who designed your car.” Remember that one of the key factors in the Explorer/Firestone fiasco that brought us the TREAD Act was exactly the opposite.
In the case of the Explorer, Ford posted lower pressures than recommended by Firestone in order to achieve a better (more car-like) ride. Ford’s engineers discounted the tendency of US drivers to ignore routine maintenance and the greater risk of under-inflation brought on by the lower initial pressure settings.
And as an FYI the 2002 TREAD Acct required the Maximum Permissible Inflation Pressure to be shown on sidewalls.
Wow, you guys ARE smart.
A. ) Yep, it was a completely cosmetic upgrade. Hated the shiny chrome optional wheels and the fender-to-tire gap. And I needed tires.
B.) Dropped the PSI (aka lbs) to 34 all around. Seems fine.
C.) In some ways, the Super IS an LS-X swap.
Thanks!
Jim
A tire’s load carrying capacity is roughly proportional to the volume of air inside the tire. So, as the diameter increases and the aspect ratio decreases, the capacity goes down. Europeans have been using high pressures to counter this effect for years. Also as the aspect ratio goes down the tires capacity is derated for better performance. Check out the Tire and Wheel Handbook for details (of course which no one owns) but is the gospel of the industry.
David Holzman :
November 2nd, 2009 at 10:01 am
Sure, it’s less contact patch, but presumably, higher friction (due to higher weight per sq inch), so I don’t think I’m losing any grip.
—
I always thought that TTAC’ers are better auto educated than this. Here is my explanation, I hope you can at least understand it.
There are two types of friction associated with tires: static or sliding.
Under normal conditions, only static frictions occur. This force is variable, typically proportional to the thrust or braking force. And it has nothing to do with tires.
However, when this static friction grows large enough, it may cross a threshold and become sliding friction (as in wheel spin or drift). The threshold is calculated as:
weight of car x coefficient
Here, weight of car = contact area x weight per sq in
So, essentially what you said was that the weight of the car is unchanged no matter what the tire pressure is. Which is true, but not the whole picture, because the “coefficient” part is also proportional to contact area. Therefore you have practically lowered the threshold of sliding friction (i.e. out of control) by making contact area smaller.
But of course, there are other factors such as heat (as discussed above). From a strictly friction force point of view, smaller contact area is worse than a larger contact area in terms of car’s capability.
The air pressure values on the door jam rule. Period, providing the size is at or near the vehicle manufacturer’s recommendations.
The values on the tire alert you to the maximum tire capabilities to assist you in choosing an appropriate tire for your vehicle.
THE DETAILS
If you read it, it’s actually a sentence. On my tires it says “Maximum load 1356 lbs at 51 psi”. Some cars will have different values for light and at maximum load while others will state “Standard inflation values for all loads”.
The door jam says 32 psi front and 29 rear. These tires have the capability of being inflated much higher than the car manufacturer’s maximum recommendations.
The door jam says “Gross Axle Weight Rating-Front = 2359 lbs.” and “Gross Axle Weight Rating-Rear = 1918 lbs.” These tires have the capability of being loaded much higher than the car manufacturer’s maximum recommendations (car = 1180 lbs vs. tire = 1356 lbs.).
The tires, Pirelli P-Zero Nero, provide an additional margin of safety when loaded and inflated properly per my vehicle manufacturer’s recommendations. They would be a poor choice for a vehicle with a Gross Vehicle Weight Rating of 5425 lbs or more as, even if inflated to 51 lbs, one or more tires would be overloaded.
You could just buy a $12 lighter plug in tire inflator at pep boys, drive around a bit at different pressures and see what you like. That’s what I do. I would start with what’s in your manual as recommended and go from there. 40 is too high for sure. The higher the pressure, the less grip, the harder the ride. Below 30 and you start to feel too squishy. When I had a fwd vehicle I used to bump the back tire up a couple psi to get less grip in the rear and have a little oversteer on command.
I should add that the tire inflation pressures on the door jam rule if the size (both diameter and width) are the vehicle manufacturer’s recommendations.
If you are changing to something different, depending on how far you go, your new tires still have to meet the weight rating but the inflation pressure becomes an unknown an expert will have to calculate for that tire in your application.
I have always run plus-sized low-profile tires at higher pressures than the default because of one overriding concern — potholes. The plus-size tires are usually mounted on expensive alloy rims that will not bend, but shatter if the rubber deflects too far. Except for cheapo discount rims that can be discarded along with the damaged tire, the cost of 300+ clams a pop (har-har) for replacement metal points to the need for a little more stiffness. I run 245/40x18s in the summer at 37 psi; the 235/45×17 snows are set to the default 35 psi on a Volvo S60R. So far, no lost rims.
TomH : With regard to your comment that sidewall pressures “are usually (always?) the maximum pressure allowed by the tire maker, with little relevance to the engineers who designed your car.” Remember that one of the key factors in the Explorer/Firestone fiasco that brought us the TREAD Act was exactly the opposite.
Very good point. I shoulda remembered that, all things considered. I always wanted to get ahold of all the accident information during that lawsuit, in particular:
1. Fatalities vs. Seat Belt usage
2. Condition of brake pads/shoes in question
3. Tread depth of tires
4. Cold tire pressure of affected vehicles…you know, of the remaining three tires that didn’t explode.
Sometimes I wonder if tire pressures were a moot point, because tire quality (Traction B, Temperature C? Really?) was my biggest concern. Ford’s beancounters really screwed the pooch there.
Sajeev,
Temp is partially a function of inflation pressure, as an under inflated tire will flex more and as a result runs at higher temps. (Flex a coat hanger back and forth several times and feel the flexed area with your hand to get some empirical evidence.)
Lots of guys screwed the pooch on that one as it ruined one of the deepest historical partnerships of the modern industrial era.
TomH :
November 2nd, 2009 at 1:54 pm
Sajeev,
With regard to your comment that sidewall pressures “are usually (always?) the maximum pressure allowed by the tire maker, with little relevance to the engineers who designed your car.” Remember that one of the key factors in the Explorer/Firestone fiasco that brought us the TREAD Act was exactly the opposite.
In the case of the Explorer, Ford posted lower pressures than recommended by Firestone in order to achieve a better (more car-like) ride. Ford’s engineers discounted the tendency of US drivers to ignore routine maintenance and the greater risk of under-inflation brought on by the lower initial pressure settings.
And as an FYI the 2002 TREAD Acct required the Maximum Permissible Inflation Pressure to be shown on sidewalls.
I bought a new 1994 Mazda Navajo (Explorer clone) when Explorers were all the rage – before the Firestone fiasco. And yes, it came with the infamous Firestone tires. The sidewall said 35psi max. However, the door jamb/owners manual said 26 psi recommended. Now 26 psi seemed awful low to me, but I did 26 psi anyway – because that’s what the book said. The vehicle rode and felt too vague and mushy, it felt like the parking brake was left partially on, and the tires wore out fairly quickly – not to mention crappy fuel mileage. I felt like I was riding on 4 marshmallows. And this was cold pressure first thing in the morning before the vehicle had moved. I experimented with different cold pressures and finally decided on 30 psi as a good compromise – granted, I’m no Ford/Mazda engineer, but it was mine.
By 1998, the Firestones’ sidewalls were dry rotted and the tread was getting thin. I got rid of the Firestones (about 2 years before the big recall) and replaced them with Big O tires. The Big O people recommended 30 psi, not 26. I kept them at 30 and I confirmed that the ride, handling, and mileage was better than at it was at 26.
Somewhere I remember reading that Ford designed the Explorer to replace the Bronco II because of all the bad PR the Bronco II got from rollover accidents. One way Ford hedged their bets on the Explorers NOT doing the same was by lowering the tire pressure to lower the Explorer’s center of gravity. Reportedly, Ford wanted to lower the tire pressure to 22 psi. Firestone said no way – 30 psi is our strong recommendation. And so the story goes, they made an uneasy compromise at 26 psi with neither side happy at this decision. And look what happened.
At least six-sided standard dice can be used by engineers to determine the psi to place on the label rather than the more expensive dice with more than six sides.
Sorry to pick nits here, but it’s called a “door jamb”.
A “door jam” is what happens when you forget to remove fingers, toes, or other appendages before closing said door. Hopefully you walk away with all of your pieces/parts still attached.
I just went through this hub-bub on my CTS, which while it is not the CTS-V, it still has a (newly-refresehsed) sports suspension. I did what the garage folks did; kept the tire pressure at the rating on the tire’s walls. I found the ride to be extremely harsh however, and after bringing it down to the OEM (GM’s) limit of 30PSI, the ride became tame once again.
Don’t forget, you’ll find the auto-manufacturer’s tire pressure rating on a sticker in the driver’s side door siding.
Somewhere I remember reading that Ford designed the Explorer to replace the Bronco II because of all the bad PR the Bronco II got from rollover accidents. One way Ford hedged their bets on the Explorers NOT doing the same was by lowering the tire pressure to lower the Explorer’s center of gravity. Reportedly, Ford wanted to lower the tire pressure to 22 psi. Firestone said no way – 30 psi is our strong recommendation. And so the story goes, they made an uneasy compromise at 26 psi with neither side happy at this decision. And look what happened.…
If I recall correctly, the lower pressure was not to drop the center of gravity, but was to make the vehicle understeer more easily at lower speeds. This was supposed to reduce high speed rollovers. This from the company that increased the size of Bronco II gas tanks to lower that vehicle’s center of gravity to reduce rollovers, all because they were behind the 8 ball with Chevy’s release of the small Blazer.
Porsche986 and Carperson are right, IMHO. Every car sold in US for several decades has factory tire pressure sticker. VAG puts them inside fuel filler door. VAG gives different amounts for different loads, so yes, the weight on the tire is a strong factor. Mazda just gives one number on Miata with its 350 lb gross payload.
So, do what Buick says.
Run very fast from any dealership goon so ignorant as to cite burst pressures on the tire.
I had a VW dealer goon do a walk around on new Jetta, telling me that brake proportioning valve is just like ABS. I said, “thats nice can I go now”.
The burst pressures are understated, i.e. have safety factor. Go ahead and exceed them if you have some reason. Less traction, concrete tire ride quality, wearing out center tread, increased cord damage on impact are among downsides.
johnny ro :
November 2nd, 2009 at 9:58 pm
Run very fast from any dealership goon so ignorant as to cite burst pressures on the tire.
Please, they aren’t “burst pressures”. That’s simply not true.
The max load/inflation rating on the tire is simply the point at which the performance envelope (grip) begins to go significantly negative at max loading. It’s an asymmetrical bell curve, actually, with the longer slope on the low side of the “max load” pressure rating.
It’s also the COLD inflation pressure that’s listed in the spec, so the tire would naturally see much higher pressures once hot, or during dynamic loading, so the idea that a passenger tire could “burst” by running rated max load/inflation pressure is simply ludicrous.
Yes, tires will feel much firmer at higher pressures. Whether they have less traction or not at higher pressures (compared to the door jamb spec) is entirely dependent on whether the tire was sufficiently inflated to begin with – in the vast majority of cases, tires have the most lateral grip at a pressure value that is higher than the best “comfort” compromise value, which itself is usually higher than the door jamb spec.
Wearing the center tread is not an issue if the tires are not over-inflated for the load conditions. A tire is not a rubber balloon, after all, it’s an engineered composite toroid. Tires don’t bulge with air pressure, they stiffen. On most modern tires, on most modern (3000lb+) cars, it would require pressures significantly higher than an ideal comfort/grip compromise pressure to generate the characteristic center tread wear that you suggest. Pressure in the mid to high 40’s, at the least, which no-one has suggested here.
So my little CUV calls for 26 psi cold. Unloaded. How much do I raise the tire pressures when it is fully loaded? Is there a sliding scale? Say max cargo weight means 32 psi. At half the max weight do I use 29 psi?
FWIW I use about 2-3 psi more than the door jamb calls for. Rides a little better (tighter) and the fuel mileage is a little better. Tire wear is a little better because we aren’t sawing the edges off of the tires from cornering.
joeaverage : FWIW I use about 2-3 psi more than the door jamb calls for. Rides a little better (tighter) and the fuel mileage is a little better. Tire wear is a little better because we aren’t sawing the edges off of the tires from cornering.
Since 26psi is pretty low by modern standards and you are enjoying 29-30 psi, I’d keep it there. Check tire wear over the course of years, not days or months to know for sure.