Tire Pressure Confusion

SEMIJim

Original poster
Member
Apr 13, 2013
116
My TB has Goodyear Fortera HL P245/65R17s. The door jamb says 30 PSI front and 35 PSI rear. The tires currently have 38 PSI all around.

Thing is: Looking at my wife's 2003 TB vs. my truck: It's apparent my tires are at least much wider, so I'm wondering if the door jamb specs are correct. I'm not certain what I should do.

I've been driving the truck for about six weeks, city, highway & expressway, and it rides and handles great.

So... what says the wisdom of the site?

Jim
 

Playsinsnow

Member
Nov 17, 2012
9,727
The specs on the door jamb came from the engineers at GM. It is wise to stay near the recommendation for longer tire life and vehicle handling.

That being said, you should also consider the conditions for your drive. Highway 38psi is a lil stiff for me. I like 35 in back, 32 psi up front. I had my inner treads wear faster at 37. If you drive in high heat, you may want to drop it a little. Also consider how high the tire max psi is rated to.

It is all a matter of preference. Too little (softer ride) and your tires will wear faster on the outer tread, too much (firmer ride) the middle will wear and go bald faster. Remember that's all relative to your tire though. 32psi can feel like 36psi in another. I have 32 in summer, 35 in winter up front. 35psi in rear. I stretched 60k on a 50k A/T tire rotating every oil change

When in doubt, stick to the owners manual for peace of mind. Years can change. I wouldn't refer to a different model year for my own. My .02
 

dmanns67

Member
Apr 3, 2013
32,979
Ohio
I usually try to keep 35psi in the front and rear and have had good luck with tread life. All of my tread wears evenly. I also make sure to rotate the tires every 6,000 miles and usually check the psi every two weeks, unless I get a warning on the DIC.

When I put air in my tires I make sure my TB has been sitting so I can get an accurate reading. As you know when air molecules heat up they expand, which in turn increases the psi. I personally like the firmer ride and steering does not feel as sluggish. Will also help with MPGs.

@Playsinsnow, because the engineers said so lol. Sounds like what my parents use to tell me when I was 5.
 

Playsinsnow

Member
Nov 17, 2012
9,727
dmanns67 said:
@Playsinsnow, because the engineers said so lol. Sounds like what my parents use to tell me when I was 5.

lol
I can only assume it helps put a larger contact point with a slightly lower psi helping to reduce body sway when turning/towing? Or the extra heat upfront warms em up?

That's the only answer I've been given. Just like shift into to 4wd whenever (yeah right not anymore) and to use only ac Delco plugs. Pick your battles I guess! I def notice mpgs drop when front/rear is under 30psi. And these are all cold checks when sitting for 30mins. Overnight is better.
 

Chickenhawk

Member
Dec 6, 2011
775
Keep in mind that door sticker pressures are designed to be SAFE; they are not designed for the best handling.

Ma and Pa Kettle will be fine at door sticker pressures. The rest of us may want to maximize our traction and performance and potentially use pressures to balance out the handling a bit.

Some people feel door sticker pressures don't apply if they have changed from OEM. This is not true. Door sticker pressures will almost always be safe, no matter what you put on there. Unless it is some made-in-China counterfeit import (and there are a lot of them out there - all with very North American-sounding 'adventurous' names) they just don't make tires in our correct size that would be unsafe at those pressures.

Ah, but what if we want not just SAFE, but the BEST pressure for our individual purposes. A rule of thumb is that a P-size tire develops its best traction at around 80% of maximum sidewall pressures. Your maximum sidewall pressures are 44 PSI, so therefore, your best traction is at 35 to 36 PSI. Now, you need to determine your own individual driving style, the load you traditionally carry and how far you want to push handling.

Me, I would say 30 is too low for the front. If I corner hard, the tires would tend to 'tuck under.' I want maximum traction in the front to minimize the designed-in understeer of these vehicles, and I want to prevent tuck-in in an emergency, so I am running my fronts at the high end of the maximum traction plateau.

Now the rears. I don't carry big loads, plus I can always go higher (up to 44 if necessary) when carrying a load. I want a bit of a softer ride, and the rear tire pressures contribute more to that ride than the front in most cases. Therefore, I am going toward the softer end of the traction plateau. So what would I run for my driving style with those tires? 35 front; 33 rear when lightly loaded, and 35 front; 40 rear when carrying a load. (I don't corner at 9/10s with a heavy load in the back because that would be dumb.)

Now understand that is me. Everyone is different. I am very sensitive about things like dynamic balance and traction circles, and I even went to a slightly stiffer rear sway bar just to make the handling of my truck more neutral.

So i would suggest you try coming down in 2 PSI increments for several weeks and see which one you like the best. I agree that 38 is pretty high. I am willing to bet you are going to be much happier at 33 to 35 front and rear.

(By the way, door sticker pressures have changed over the years, not because our platform has changed but because the philosophy has changed. Companies like GM realize that the vast majority of drivers are stupid, and if they say 30 rear, but pump them up to 38 when carrying a load, Ma and Pa will leave them at 30 forever, even when taking 1000 pounds of gear and four friends home from the cottage at 140 on the interstate. By specifying 35, they are better covering themselves for the majority of the crappy drivers who wouldn't know the difference between understeer and oversteer if it reached out from a corner and hit them over the head with the armco.)
 

gmcman

Member
Dec 12, 2011
4,656
I have had the best treadwear at 35 F&R. After having the envoy weighed I keep the front at 36 and the rear at 35, since it's heavier in the front and for some reason I feel happy that there's one more PSI in the front. One PSI does make a noticeable difference in the appearance of the sidewall, I can't feel it but it's obvious. If I'm loading the trailer then I take the rear up to around 38-40.

It's important to keep up on the tire pressure as it changes frequently with the change in ambient temps. Take a reading with the outside air at 35 deg then at 85 deg and you will be surprised. I take a reading in the mornings of the weekend before the sun hits the side of the tire, I try to do this every weekend.

I feel 30 is too low and I also believe that when you test drive one of these you want a smooth ride and 30 will surely provide this. I'm sure it's a safe pressure I just don't personally care for that low of a setting.
 

SEMIJim

Original poster
Member
Apr 13, 2013
116
Okay, thanks for the feedback, guys!

I suppose that sometime later I'll do a chalk test. That appears to be the only way to know for sure. In the meantime I think I'll lower them all to 36 PSI - which is a compromise between where they are and the "average" of what most are recommending.

Jim
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
Do a chalk test
 

meerschm

Member
Aug 26, 2012
1,079
so what is a chalk test?

are the tires installed the same size as oem?

my 02 has 245/70-16

I run at 32 PSI front and back. I find higher pressures too harsh.

and the poo poo of rated pressures is, well, to each his own.

ratings are a balance, not only for safety, but ride quality, and longevity.

over inflated tires wear improperly, as do underinflated.

the allignment and proper shocks have impact on tire life, as well as ride quality.

free advice, at least you get what you pay for :smile:
 

dmanns67

Member
Apr 3, 2013
32,979
Ohio
meerschm said:
so what is a chalk test?

The chalk test is an easy way to determine what psi is best for tread wear.

If you're not familiar with this, color the tread area with chalk. Drive a couple hundred yards and look at how the chalk wore off.

More wear in the middle = too much psi. More wear on the outside edges = not enough psi.
 

Sparky

Member
Dec 4, 2011
12,927
I have been running 35 front and rear on mine. The lower PSI rating in the rear annoyed me. I've also gotten 60k (and counting) out of my 50k rated A/T tires with exceptional wear patterns so I'm sticking with that :biggrin:

Now if/when I go a non-stock size I'll have to fiddle I guess.
 

RayVoy

Member
Nov 20, 2011
939
Sparky said:
I have been running 25 front and rear on mine.
Is that a typo, and you actually mean 35 front and rear?
 

Sparky

Member
Dec 4, 2011
12,927
Oops. Yep, 35 PSI.
 

RayVoy

Member
Nov 20, 2011
939
25 would cost a lot of gas and a lot of rubber.
 

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