Tire pressure calculation

Stockli said:
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?
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HI I know I'm late to the party on this one and I'm not really smart... but how did you get 24.5psi ??
(4450/2270x4)50 = 392.07
Am I missing something?
 
What if your jeep doesn't weigh 4450#? Most are heavier than the published curb weight. My LJ is 4720# as currently equipped, full tank of gas, me, but no gear - its true curb weight as built. [Note: some/(most/all?) manufacturers allocate 150# for the average driver in published curb weights.]


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Another method for determining tire pressure when changing sizes from stock to larger is to use this calculator, which is based upon a spreadsheet derived from the Tire and Rim Association Load Inflation tables:

https://tiresize.com/pressure-calculator/

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If you would prefer to calculate on your own the appropriate starting tire pressure for your new tires based on the actual weight of your vehicle, the TRA Load Inflation Table is here:

https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf

Accurate results will require getting your jeep weighed, front axle + rear axle, with a full tank of gas + your gear + you. (The total may surprise you.) If you don't have access to a public scale, then use the jeep's GVWR and approximate.


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BTW, weighing my jeep and then using the TRA tables suggests tire pressure of 26 psi for my LT315/75R16-E Mickey Thompson Baja ATZ P3 tires.

For comparison, when I purchased BFG KO2 LT265/75R16-E tires for my Sprinter, which weighs almost exactly its GVRW and was originally equipped with LT245/75R16 tires, the tire pressure calculator showed recommended pressures of 42F/60R rather than the 49F/70R shown on the door placard for stock tires. I then called BFG and asked them what pressures I should run with the larger tires. The answer, not coincidentally, was 42F/60R. Seemed low for that much weight, and I'm sure there will be more gear/weight in the future, so I run 45/65.
 
Mr. Bills said:
[Note: some/(most/all?) manufacturers allocate 150# for the average driver in published curb weights.]
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That is probably close to being, if not exactly, true, and I think they're way off. My son and daughter both look like some kind of telephone poles but the two of them together average about 180# each, mainly due to his height. Don't even ask about how I compare to them.
 
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Hey guys!
I just sold my tires and rims.
I am finally happy to run some new 15" rims (instead of 16"), with some 31"x10.5" Bf Goodrich KO2s.
Is 29 psi the correct pressure for running them around town?
 
I’ve read the threads, done the math and tried 28-29 for a year. The steering felt a bit heavy, the gas mileage lower, handling so so. Recently I have been experimenting and find 32 lbs to be to my liking. I air down when off road so no impact there.
 
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TJim said:
Are you a friend of Greece?
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I am! Oops, wrong spelling.;)
Grease.jpg
 
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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 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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Here’s another tire related question for you, Jerry. My TJ has 33/12.50 KO2 tires that are 5 years old so I’m changing them out in about a year just due to age. I’m thinking about keeping the 33” diameter, but would like to get a narrower tire; 10.50 or 11.50 (if I can find that size). I’m thinking that the lower unsprung weight & less rolling mass would make it drive even better on the road & narrower tires would tuck up under the fenders better so the Jeep would generally stay cleaner for everyday driving.
I‘m running 26 psi in the 12.50 tires. What should I run in 11.50s or 10.50s? I’m guessing 28-30 psi.
Thanks..
 
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Mudflat said:
Here’s another tire related question for you, Jerry. My TJ has 33/12.50 KO2 tires that are 5 years old so I’m changing them out in about a year just due to age. I’m thinking about keeping the 33” diameter, but would like to get a narrower tire; 10.50 or 11.50 (if I can find that size). I’m thinking that the lower unsprung weight & less rolling mass would make it drive even better on the road & narrower tires would tuck up under the fenders better so the Jeep would generally stay cleaner for everyday driving.
I‘m running 26 psi in the 12.50 tires. What should I run in 11.50s or 10.50s? I’m guessing 28-30 psi.
Thanks..
Click to expand...
It makes sense to bump it up a psi or two due to the smaller volume of air in that narrower width tire. :)
 
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