Tire size help.

[skokie]

What would be my tire size with a 31-10.5 x 15 tire? From what i was able to find out the stock size would be 235/75 x 15. Is this the same as the 31-10.5 x 15?

Thanks

[toddtheodd]

235/75 x 15 is a 29 (28.87995) inch tire.
31-10.5 x 15 is 31 inches.

If you want to stay pretty close to the 31 inches, go with 255/75 x 15... that is a 30 inch tire.

Todd

[skokie]

I want to eventually get back to a 235/75 tire. These are the tires that the previous owner had installed before I bought the truck. They have about 15000 miles left on em so I want to use them up. The problem that I have now is at 20 mph I am in reality going 25. So I am 20% off and I am not sure if going to a smaller tire size will fix that or not. I need to ask the tranny guy if he took the 400 turbo back to stock on the drive and driven gear. I pulled the speedo housing tonight and found a 37 tooth gear. So if the calculator is right I would need a 45 tooth with the drive gear in there now.

[toddtheodd]

Yes, going to a smaller tire size will help that.
I am currently running a 27 inch tire after running 31 inch tires. Not only is my speedometer accurate again, but I have more acceleration. It's amazing how much of a difference 4 inches made.
I believe that factory tires were supposed to be 28 inches. Somebody correct me if I'm wrong.
My tire size by the way is 255/60 x 15, and these fit just fine on the stock rims.
Todd

[skokie]

So assumming that the tranny shop set it up for a 27 inch tire that would explain why my speedo is off by 20%. What size tires are 27 inches anyway?

[toddtheodd]

If the tranny shop set it up to factory original specs (with a factory gear ratio in the rear end) the stock size tire would be the one's you want. 235/75x15
My tires are 27 inches and they are 255/60x15.

[4x4Poet]

Tire diameters can vary among mfrs and even lots. 31x10.50x15 tires are usually around 30 inches tall. Most accurate diameter is when tire is inflated to street pressure and unweighted.

[Tx Firefighter]

Skokie, like was said, stock tires are a bit over 28 inches tall. A 235/75/15 works well, or my favorite is a 275/60/15. They are also 28 inches tall, but much wider.

It depends on what "look" you want for your truck.

By the way, don't get too caught up in having a tire exactly 28.8 inches tall. Like the poet said, tires vary a bit from brand to brand. Also, believe it or not, a fresh tire with full tread is a good bit taller than a tire that is worn from a lot of miles. Those perfect tires will be a 1/2 or 3/4 inch shorter when they get worn some.