Tire Pressure Calculation

[RichardSenn]

I know I've seen it somewhere and I didn't find it searching, but other than trial and error is there a mathematical formula for figuring tire pressure?

I just bought new tires but bought a bigger tire than OEM. Went from 265/70/17 to 285/70/17 on my Chevy Silverado. The original tires wore very evenly at 36 PSI as the door sticker stated. I've put about 200 miles on the new 285's at 36 PSI, and although it's not a high tech method it appears the centers are wearing much quicker just by looking at those new little nubs on the tires. Tried a chalk test as well which confirmed the edge of each side of the tire tread was not contacting the ground.

Does it stand to reason that since the new tire has a higher load rating that I would need slightly less PSI? At any rate I lowered each tire down to 33 PSI to see how that does.

 

[The Critic]

There are load tables that can be used to calculate the optimal tire pressure for your tire size and application.

 

[Colt45ws]

Are the new tires P or LT? Are the old tires a P as well?

 

[CapriRacer]

Originally Posted By: RichardSenn
I know I've seen it somewhere and I didn't find it searching, but other than trial and error is there a mathematical formula for figuring tire pressure?

I just bought new tires but bought a bigger tire than OEM. Went from 265/70/17 to 285/70/17 on my Chevy Silverado. The original tires wore very evenly at 36 PSI as the door sticker stated. I've put about 200 miles on the new 285's at 36 PSI, and although it's not a high tech method it appears the centers are wearing much quicker just by looking at those new little nubs on the tires. Tried a chalk test as well which confirmed the edge of each side of the tire tread was not contacting the ground.

Does it stand to reason that since the new tire has a higher load rating that I would need slightly less PSI? At any rate I lowered each tire down to 33 PSI to see how that does.


Yes, there is a formula, but it is so complex, it is usually expressed as a table - commonly called a load table.

The problem is that these tables are rarely published on the internet because they are copyrighted - and they are the only source of revenue for the tire standardizing organizations.

You could buy a copy, but they are $60 each!

The good news is that I have a copy.

So if your original tires were P265/70R17 at 35 psi (I corrected that for you!), then a P285/70R17 has the same load carrying capacity at 28 psi, and for an LT285/70R17 it is 45 psi.

 

[RichardSenn]

Originally Posted By: CapriRacer
Originally Posted By: RichardSenn
I know I've seen it somewhere and I didn't find it searching, but other than trial and error is there a mathematical formula for figuring tire pressure?

I just bought new tires but bought a bigger tire than OEM. Went from 265/70/17 to 285/70/17 on my Chevy Silverado. The original tires wore very evenly at 36 PSI as the door sticker stated. I've put about 200 miles on the new 285's at 36 PSI, and although it's not a high tech method it appears the centers are wearing much quicker just by looking at those new little nubs on the tires. Tried a chalk test as well which confirmed the edge of each side of the tire tread was not contacting the ground.

Does it stand to reason that since the new tire has a higher load rating that I would need slightly less PSI? At any rate I lowered each tire down to 33 PSI to see how that does.


Yes, there is a formula, but it is so complex, it is usually expressed as a table - commonly called a load table.

The problem is that these tables are rarely published on the internet because they are copyrighted - and they are the only source of revenue for the tire standardizing organizations.

You could buy a copy, but they are $60 each!

The good news is that I have a copy.

So if your original tires were P265/70R17 at 35 psi (I corrected that for you!), then a P285/70R17 has the same load carrying capacity at 28 psi, and for an LT285/70R17 it is 45 psi.



Awesome, thanks CapriRacer. I could of sworn it was 36 PSI so I just went and checked the door sticker and sure enough you are right 35 PSI. I guess I was just thinking 36 because that's what I always set them at.

I may bump them down to 28 PSI and use that as a starting point. After my chalk test I could tell I needed to lower the PSI but I would have never thought it would be that low. I was thinking somewhere around 32 or 33 PSI.

Oh, and yes both tires were passenger tires not light truck tires.

 

[johnsmellsalot]

My brother's Explorer P235/75/R15 takes 26 PSI. My mom's Civic P205/55/R16 takes 32 PSI. Both are OEM sizes and pressures.

I would try 30 PSI at first and see if the edges make contact. If not, lower it another 2 PSI.

 

[SteveSRT8]

The Exploder is a poor example as the pressure is artificially lower to help rollover.

 

[Colt45ws]

Google "toyo load table" It has everything but LT-Floatation tables.

 

[mechtech2]

Varying tire construction will blow the math away.
You new tires have more 'overhang' on the rims, and are taller.
Also, they are sure to be wider. Internal construction? Also different.

A cheap depth gauge is your best bet, here.
And forget about 200 mile tire readings. You need TIME.