Dave writes:
While I understand that recommended tire pressure is located on the door jamb in a car, what happens when the factory tires are replaced, upgraded, down graded, replaced with used or even recaps where available (are re-treads even available?), different tread and wear and heat ratings?
What I am trying to ask is: are the manufacturer’s recommendations applicable only to the factory tires, or to non- original brand replacements as well?
Sajeev replies:
I haven’t seen passenger car tire re-treads in well over a decade, but that won’t change the answer: the factory pressure setting always applies.
Well, almost. I can think of a few exceptions: dynamic tuning in racing conditions, door jamb stickers on Nasser-era Ford Explorers, and a somewhat-misguided need for a smoother ride (lower) or fuel economy (higher). And maybe, possibly, from changing over to wheel/tires with a significantly different sidewall height.
Let’s focus on that last one. If you live in an area with terrible roads, but absolutely must have twankys on your Corolla and 26-inch rims on your Silverado, you might up the air pressure a good 1-5 psi to help low profile tires fight a bent rim. But that is a long shot, the extra air probably won’t matter to your run-of-the-mill frost heave.
So stick with what’s on the door jamb.
[Send your technical queries to mehta@ttac.com]
Increased tire pressure for a tow vehicle or when carrying a heavy load can improve directional stability. The weight of the load compresses the tires making them look, and perform like they’re partially flat (ie lateral movement).
“And maybe, possibly, from changing over to wheel/tires with a significantly different sidewall height.”
I have always wanted to know if there is a set theory/rule of thumb for this. For example, I upped the size of tire my FJ runs from the stock 265/70/17 to 285/70/17. The factory specs state to use 32psi but I’ve always used 35-39 with both tires just for the hell of it. I’ll be interested to see what others comment.
DrBeets
Two things happened when you changed the tire size. One, your speedometer is off low – you are going faster than indicated. Two, if you ever do load down that FJ with a lot of heavy junk, you increase the risk of the tire rubbing the fenders or some other bits down there. For a good explanation of how to plus or minus size tires, check tirerack.com
In your case you have increased the rolling diameter by (285-265)x(70/100)x2 mm or, in Christian units, 1 3/16″. But the increased sidewall height should give a softer ride, other things equal.
Your preferred choice would be a different tire/rim size.
I think an interesting follow-up question would be what tire pressure should you use if your car originally came equipped with bias ply tires and you’ve since upgraded to radials.
If you have put 20 inch rims on your Corolla I highly recommend putting your head right next to the tire and inflating it to 200 psi.
no_slushbox: Thanks for a hearty laugh.
God and to think I used to like the style of that era of Cadillac.
Thanks for the wisdom, Piston Slap.
MC5 reference!!
Sweet!!
Sajeev, YOU ROCK!
DrBeets : I’d like to see if the bigger tires (and not done the correct +1 way) with more air pressure last as long (tread wise) as the stockers. I’m guessing they’ll wear out faster.
educatordan: stick with the 1980s and older models. The newer ones are only good for their suspensions and powertrains.
BlisterInTheSun: I have my moments. Not so easy when you do a daily column that tries to be factual at the same time. Let me tell you!
What I am trying to ask is: are the manufacturer’s recommendations applicable only to the factory tires, or to non- original brand replacements as well?
What I’m trying to figure out is WHY? Why on God’s green earth would you waste the time and money to create such an ugly machine that is as utterly useless as to make it yard sculpture? How bout a call to stop punishing Corolla’s and ancient Impala’s with 26″ spinnin’ wheels.
I always fill the tires on my Accord ~5-8 extra lbs (30 is on the door jamb), to improve gas mileage and grip. I figure the mfgs recs emphasize smooth ride; I’m more interested in fun. With the higher pressure and the Bridgestone Potenzas, I’ve got it in spades (normal size tires).
@ celebrity208
Most of my cars (not my miata :) ) have two sets of inflation recommendations: one for lightly loaded (1-3 pax), one for fully loaded or speeds in excess of XXX mph.
So I’m agreeing with you, I usually just follow those recommendations.
Unrelated to your comment: the miata world is chock full of tire pressure recommendations (usually higher than the door sticker recommends). I’ve always stuck with the factory recommended 26 PSI on my NA and still smile from ear to ear when driving it. Maybe I’m missing out on something.
I try to follow the factory recommendations, but sometimes they don’t make sense. My F150 tells me to use 43psi on the rear tires, but the new rubber says “Max 40 psi”. What to do? I run them at 34-35psi to allow for some heating.
What kind of tires did the truck come with new? No idea what they were thinking in 1993…
OK, B’n’B, how about this: Take a ’75 LBC (that came with radials, originally) and look at the factory specs that ask for 20/22 psi and try to mate new’ish tires that ask for 30-35 psi.
Who do you trust? 1970’s technology or the manufacturer? The car weighs 2600 lbs, (less rust: add Bondo).
Chuck G., anyone, any thoughts?
According to physics
Pressure / Temperature = constant
with Temperature in kelvin
You can go much closer to 40 psi to have a save margin for heat.
I used to run my tires at the door placard pressure, but now I run them at the sidewall max pressure on the tires, 44psi.
Why? Well, on passenger cars, steel belted radials don’t bulge out in the middle of the tread like they want you to think. The manufacturer of the tire is gonna know more about that particular tire than the car manufacturer. So if they say it’s safe to go up to 44psi cold, then that’s where I’m going.
What happened once I did that? Fuel economy increase of about 3% is one thing. Other than that, the tires are supposed to last a little bit longer.
Final thoughts, you’ll be fine at door placard pressure, but what about a month later when your psi’s dropped to 26. That’s when you start getting sidewall flex, and that’s dangerous.
wannabewannabe: That’s what I have wondered since putting radials on my 63 Valiant. I usually keep them at 34 psi. The Bias plys were something like 28. Maybe.
It’s been so long now I can’t remember.But the change to radials makes the bloody thing grip like a tractor.
I avoided the change over because many years ago I was told that “putting radials on a car that wasn’t designed for them would destroy the steering system”. That must be one for the “urban legend” files. What did I know, I was just a kid when I heard that.