Tire Pressure formula (simplified)

[SlimJim]

So i got fed up with trying to find a definitive answer of what PSI i should run my tires. Everybody just says use the door placard but that doesn't work when you've changed tire and wheel size. So instead of doing a trail and error or best guessitame i went ahead and found a local tire guru at 208 TIRE here in my town. This is how he broke it down for me on my tahoe, but it can be used for anybody changing tire sizes.
Im running 33x12.50r18 Atturro M/Ts with Moto Metal 970s

EXAMPLE:
MAX TIRE LOAD= 2910 pounds

MAX TIRE psi= 65 psi

GAWR Front & Rear (vehicle weight front and rear, found on door placard)
FRONT=3600
REAR= 4000

So you take your tires MAX TIRE LOAD and divide it by the tires MAX PSI
2910/65=44.7 rounded up to 45

Then you add the GAWRs together
3600+4000=7600

Then you take the combined weight and divide by 4 (this gives you the weight each tire must support)
7600/4=1900

Next you take the answer for that and divide by the PSI you found for the first equation
1900/45= 42.2 and round it how ever you like.

The final equation gives you the psi your should be running in each of your tires. This equation take into consideration the weight of you vehicle and the max load each tire can support and disperses the weight evenly per tire. i have been running this psi in my tires and have had great results (optimal fuel mileage and wear).Hope this helps someone.

 

[Doubeleive]

I run mine at 42, shops always want to put 36 so I always "request" 42

 

[swathdiver]

Your 200 pounds shy then Jim to cover your rear axle weight.

My truck's Gross Rear Axle Weight Rating is 4,200 pounds. The original Goodyear Wranglers @ 30 psi had a maximum weight rating of 2,135 pounds each or 4,270 pounds for both on the rear axle, a 70 pound margin over the GRAWR. The door sticker for the P-Metrics is 30 psi all around.

Like you I switched over to an LT tire and insisted on figuring out the optimal tire pressure, not trusting BF Goodrich's recommendation. I was able to obtain the LT tire's load ratings at certain pressures and graphed it and discovered that 42 psi gave me 2,150 pounds per tire or 4,300 pounds for the rear axle. Tire wear at this rating was still a little high so I eventually acquiesced and began running them at 50 psi (BFGs recommended pressure) and they ride about the same and wear has become imperceptible compared with before.

Around this time I came across the door sticker for a 2500 Yukon XL with my truck's tire and wheel size (265-70-17) for Load Range E tires. 50 PSI for the front axle and 60 PSI for the rear axle. The 2500s GFAWR is 4,180 pounds and both front tires at 50 psi support 4,940 pounds. The 2500s GRAWR is 5,500 pounds (4x4) and both rear tires at 60 psi support 5,520 pounds. The GVWR of the 2500 is 9,300 pounds and the tires aired up to support 10,460 pounds.

And now to muddy the waters, we have your example where you've not only traded P-Metrics for LT tires, you went bigger which generally equates to less air pressure. Now I'm confused! LOL

 

[Matt_]

All that is doing is assuming load is a linear relationship with pressure, and then dividing evenly amongst the tires which doesn't take into account the rear tires need more pressure when loaded.

Proper way is to look up the TRA load vs inflation tables for your tire size, then use GAWR divide by two for each axle. If using P rated tires on a truck/SUV you need to derate the carrying capacity of the tire by dividing by 1.1.

You can also use the TRA tables to find the load carrying carry capacity of the current tires then find the equivalent in the new size. Just remember the 1.1 factor if going from P to LT

 

[Scottydoggs]

i dont do math, i read the sidewall of the tires and add air.

take this math and figure out a 80,000 gvw roll off truck. answer is 100 psi, i dont care what your math says thats max psi on the tire.

the side wall of every tire made is stamped with the max pressure....whos out there making things hard?

i can just see it now, hold on guys, i need to look up my axle weights, then do some math, then i can fill my tires,,,,this is all horse shit to me.

im sure no one here is maxing out their hauling capability's in a tahoe. unless you haul lead bricks for some reason. and even then, you run all the air pressure, not 1/2 of it.


if you want your tires to last as long as they can and get the best mpg possible, you air them up to max. theres really no debate here.

 

[Sasquatch]

I run really low pressure in the rears because with my intake exhaust and blackbear tune i need as much traction as possible to keep them from spinning.

With that BS out of the way i actually run 42 psi.

 

[Matt_]

Running E rated tires at max psi on a 1500 series SUV will wear the center tread too fast, decrease traction, and increase stopping distance. Max psi on a sidewall does not apply to every vehicle, and there are negative results with over inflation.

 

[Scottydoggs]

keep in mine, if the tire fits the wheel as it should there should be no curving of the tire when at full psi to wear out the middle of the tires. i know people like to "make things fit" you know like running a wider tire than fits the wheel. instant curved tread. thats not so smart.

and soft tires dont handle better, they make the front end plow on ya and the tires squeal. thats lost traction.

now snow and mud, you can air down to make a larger foot print to get better traction, but thats not good for dry weather, so you'll need to air back up.

and the facts about a big fat tire doing better in snow is a myth. narrow tires gets more weight put on it and it grips better.

and we should all be running truck tires, not car tires, not the same animal as far as sidewalls go. when the truck is about twice as heavy as a car your tire ratings change. any real shop will refuse to install car tires on your truck.

 

[iamdub]

i dont do math, i read the sidewall of the tires and add air.

...the side wall of every tire made is stamped with the max pressure....whos out there making things hard?...

When a tire manufacturer makes a tire, they have no idea what vehicle it will be used on. The max PSI stamped on the sidewall is what that tire is rated for. The max load (weight) rating is also stamped on the sidewall. The vehicle that tire is used on is what determines the amount of air pressure required. Heavier vehicles will need more air, lighter will need less. The idea is not to have as solid of a tire as possible as this reduces traction and braking capabilities. Sure, it helps with fuel economy, but that's pennies when compared to wearing the center of the tire tread faster and reducing stopping distance in an emergency. The loss of ride comfort is the least of concern.

keep in mine, if the tire fits the wheel as it should there should be no curving of the tire when at full psi to wear out the middle of the tires.

This is partially true, but air pressure versus the load on that tire is an even bigger factor of even tread wear.

i know people like to "make things fit" you know like running a wider tire than fits the wheel. instant curved tread. thats not so smart.

Or the latest rage with running narrow, short sidewall tires stretched on wide wheels- functionally and aesthetically stupid. If you can't afford 14" wide tires, don't buy 14" wide wheels.

...and soft tires dont handle better, they make the front end plow on ya and the tires squeal. thats lost traction.

For traction (as in drag racing applications), it's the REAR tires you want soft. For racing that involves turns, the compound of tire is key, but the PSI needs to be appropriate for the loads involved. Again, not what is stamped on the sidewall is necessarily ideal. If you wanna get technical, the pressure at each corner of the car should be different unless each corner of the car bears the same loads at virtually all times. This is also why some average, non-race passenger vehicles have different PSI ratings listed from the manufacturer.

...now snow and mud, you can air down to make a larger foot print to get better traction, but thats not good for dry weather, so you'll need to air back up.

and the facts about a big fat tire doing better in snow is a myth. narrow tires gets more weight put on it and it grips better.

Not trying to pick on you, but you're contradicting yourself here:
First sentence says aired down for larger footprint = better traction, second says the "facts" is a "myth" and then says a narrow tire (aka smaller footprint) grips better.

Which is it: Larger footprint equals better traction in snow and mud or narrower footprint grips better because it has more weight on it?

...and we should all be running truck tires, not car tires, not the same animal as far as sidewalls go. when the truck is about twice as heavy as a car your tire ratings change. any real shop will refuse to install car tires on your truck.

Who's running car tires? I'm running whatever the manufacturer spent millions to have engineers design and test out to be appropriate for the vehicle to satisfy all criteria for handling, braking and ride performance. The subject here is those that have deviated from the OE design and specifications and how they can determine optimal tire pressure for this variance.

 

[Scottydoggs]

For traction (as in drag racing applications), it's the REAR tires you want soft. For racing that involves turns, the compound of tire is key, but the PSI needs to be appropriate for the loads involved. Again, not what is stamped on the sidewall is necessarily ideal. If you wanna get technical, the pressure at each corner of the car should be different unless each corner of the car bears the same loads at virtually all times. This is also why some average, non-race passenger vehicles have different PSI ratings listed from the manufacturer.

im talking about being stuck in the snow or mud or trail riding at slow speed. wake up. think rock crawlers, they run full psi? no they dont, but they also dont go over 20 mph either. unless your running king of the hammers of course, note they run a lot more air pressure for the whole course. they do a 100 mph easy. and crawl at higher speed.

drag racing, ive done drag racing, would you really run 20 psi on the hwy up to and past 100 mph? its damn sketchy at best at the track. take some turns at hwy speed, let me know how that worked out for ya.

one of my last runs with my new dr's i was of course playing with air pressure, dropped to 18 and on the back 1/2 it just started pulling to the right, wheel dead straight, had to hold it left the rest of the way till i got out the gas. FWD tq steer maybe, but other wise its a slop bucket on low psi. all its good for is to grip off the line. 21 turns out to be my magic psi at the track btw. same tires on the street get run at 40 psi. other wise it dont handle for shit.