Tire pressure calculation

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Stockli

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I am trying to get a good starting point for tire pressure for my new tires.

After calculating using the formula: (gross vehicle weight /max tire load x 4) x max tire pressure I get 24.5psi.

This seems crazy low. The tires are General Grabber at2 31x10.50x15 (max load of 2270 and max pressure of 50 psi). Gross vehicle weight of my jeep is listed at 4450 lb.

I was thinking 28-30 psi would be correct but the formula give a much lower number.

Any suggestions?
 
Put in your recommended pressure then back it up with a chalk test. Take a piece of chalk and totally cove a cross section of each tire. Move the vehicle at least couple of tie rotation and check the pattern in the chalk. I try to make pattern even as possible.
 
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Stockli said:
What do I look for after driving? Chalk to be worn except extreme edges of tread?
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Pretty much. There chalk would ideally be worn evenly across the tread. This only take several rotations to see. Any more and it tends to lose the pattern. Also do it with the vehicle loaded as you would for the majority of your driving. No sense in checking it with no cargo, empty fuel tank, and no people.
 
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The big problem I have with the chalk method is that in the hands of the inexperienced, it invariably produces an underinflated tire. Often severely underinflated... either of which is the worst thing you can do to a tire. Few tires are designed have their entire tread on the ground and trying to get the chalk to wear away evenly across the entire tread will deflated the tire more than it should be.
 
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Agreed. That is why I advocated starting with the recommended pressure and going from there. I have always started from around 36 psi. It seems to work as my wear pattern is pretty equal. Don't forget to rotate!
 
I have used the formula to figure out tire pressure in the past but always added about 3 psi to my number and ended up with 27-28 psi for 31x10.5's. I like to add a little bit more for better gas mileage. Nowadays I just set my 31's to 28 psi and once they get hot they are about 32 psi.
 
I've got mine set at 30 psi for the moment. I do have an issue though because discount tire stated they set them at 30 but my gauge said 35.

I measured in the morning before the jeep was driven. Not sure if I have a bad gauge or want. It's an old school hose and dial type.
 
Stockli said:
It's an old school hose and dial type.
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That's what I use, keep in mind that digital air pressure gouges are not necessarily more accurate than an analog gauge which is my preference for most gauges.

As Miss Moneypenny said as she shaved James Bond in Skyfall... "Sometimes the old ways are the best." ;)
 
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Since I just got new tires (33x12.5x15), I'm trying to dial in the air pressure. I started at 28 psi, and I've noticed that the little new tire nubs have started to wear off in the middle of the tires, and not the edges. That says to me that 28 psi is too much. When I did the pressure calculation, it's telling me 16ish psi, which I think is far too low.

In case I'm screwing up the calculations, the new tires are rated at 2200 lbs at 36 max psi, and my Jeep is an 05 Rubi with the hardtop on.
 
derekmac said:
Since I just got new tires (33x12.5x15), I'm trying to dial in the air pressure. I started at 28 psi, and I've noticed that the little new tire nubs have started to wear off in the middle of the tires, and not the edges. That says to me that 28 psi is too much. When I did the pressure calculation, it's telling me 16ish psi, which I think is far too low.

In case I'm screwing up the calculations, the new tires are rated at 2200 lbs at 36 max psi, and my Jeep is an 05 Rubi with the hardtop on.
Click to expand...
26 psi unloaded and driving around town to 28 psi when the TJ is fully loaded with gear/passengers has been shown to work very well on 33x12.50 tires under a Wrangler.
 
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Hi Jerry,
Tim here. I read your post replies about tire pressure. Sounds like you know your stuff. I've been having this conversation/argument for years now.
The guy that owns the tire store says 35lbs. A bunch of other folks say it should be lower. What calculation do you use?
I have a mostly stock '02 TJ with 33"/12.50-15s. When I have to brake a little on the hard side do to some nit-wit in traffic [I live in the Washington, D.C. suburbs of Maryland. We have the largest collection of traffic assholes per square mile in the world] I end up locking up the tires. They howl like a beat dog. I have the standard front disc/rear drum no anti-lock brakes. I removed the back seat when I first bought the Jeep about twelve years ago. I installed a trunk kit in its place. I think the weight difference is about even.
I usually have my 120lb Rottweiler riding shotgun. So, is there an actual calculation that you use to determine what tire pressure to run? I appreciate your guidance with this.

Cheers
Tim
 
First, the tire store owner is apparently clueless about tire pressures. Yes, and that is fairly common about tire shops. They're all about selling tires, not about becoming expert on all the various air pressures that the same tire might be appropriately filled to depending on the weight of the vehicle they'll be mounted on. Or they'll just cluelessly inflate all tires to the air pressure molded onto the sidewall as discussed below.

Two facts to know... the correct tire pressure depends on the weight of the vehicle and the tire pressure shown on the tire's sidewall is NEVER correct... that's only its maximum safe air pressure which would only be required if the tire was forced to carry its maximum safe load. Automotive tires seldom carry more than about half of their max safe load so that air pressure on the sidewall is never correct for that reason. So the exact same tire with the same exact size will have multiple correct tire pressures since it will be mounted on different vehicles with different weights.

Also, the bigger the tire, the lower the air pressure that is required. For the factory 30" tires the Jeep recommended tire pressure is only 29 psi. So that means your 33" tires will have a lower correct air pressure than the smaller factory size tire requires.

For a 33x12.50R15 tire with a Load Range C rating on a Wrangler TJ, the recommended tire pressure is 26 psi when the Jeep is lightly loaded, and 28 psi when fully loaded with people and/or gear. Experience over the years continually reinforces those two tire pressures as very appropriate for that size tire and load rating.
 
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Jerry Bransford said:
First, the tire store owner is apparently clueless about tire pressures. Yes, and that is fairly common about tire shops. They're all about selling tires, not about becoming expert on all the various air pressures that the same tire might be appropriately filled to depending on the weight of the vehicle they'll be mounted on. Or they'll just cluelessly inflate all tires to the air pressure molded onto the sidewall as discussed below.

Two facts to know... the correct tire pressure depends on the weight of the vehicle and the tire pressure shown on the tire's sidewall is NEVER correct... that's only its maximum safe air pressure which would only be required if the tire was forced to carry its maximum safe load. Automotive tires seldom carry more than about half of their max safe load so that air pressure on the sidewall is never correct for that reason. So the exact same tire with the same exact size will have multiple correct tire pressures since it will be mounted on different vehicles with different weights.

Also, the bigger the tire, the lower the air pressure that is required. For the factory 30" tires the Jeep recommended tire pressure is only something like 31-33 psi though I don't recall exactly. So that means your 33" tires will have a correct air pressure of less than the smaller factory size tire.

For a 33x12.50R15 tire with a Load Range C rating on a Wrangler TJ, the recommended tire pressure is 26 psi when the Jeep is lightly loaded, and 28 psi when fully loaded with people and/or gear. Experience over the years continually reinforces those two tire pressures as very appropriate for that size tire and load rating.
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Factory recommended pressure for 30’s is only 29 PSI.

08163FA5-BF1A-4B1E-AA23-9AC124F68934.jpeg
 
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Jerry Bransford said:
Thanks Mots, I hadn't seen a 30" factory size tire recommended air pressure sticker in years.
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You’re welcome! The sticker was upside down (by design) and hard to find when I first bought my 05. So, I took a pic once I found it and then cropped and rotated for safe keeping. I cringe when I hear folks running 35 PSI on stockers, and with 33’s...wow! Can you say “Death Wobble trigger”, LOL.
 
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