Since I have a Smarty I thought I would update my computer to reflect the larger size tires. Interestingly enough I saw the factory tire size set at 30.70 in the computer, although the stock Michelin LTX 265/70-17 were 31.4 inches in diameter. Hmmm...

Well I put on a set of BFG AT KO's (285/70-17) that were 32.7 inches in diameter according to the website. The BSW version is 32.7 and the RWL is 32.8?... Ok, what ever...

At any rate, I figured the Revolutions Per Mile (RVM) would give me a more acurate reading than diamter and used these to figure out the adjusted tire size instead. The Michelin was 657 RVM while the BFG was 636 RVM.

Let's see...

(Michelin RVM/BFG RVM) x (factory computer setting) = New setting

(657/636) x 30.7 = 31.71

Using the Smarty, I adjusted the tire size to 31.70 and checked it with my GPS. Spot On!

Has anybody else adjusted there computers for tire size? What were your results?

 

Hmmm... Looks like most people don't bother to correct their computers for the larger tire size.

Maybe I should put mine down to 15.50 and double the indicated speed. I will rack up 1/2 the miles at half that pace.

 

Well interesting that was the first math I've seen for the tire adjustment. Could have used that but instead I just used my GPS to get it to match. Found for my stock tires 31.1 matched the GPS to the truck....

 

Not everyone is up that late to reply, but there are several threads that give the formulas.
I too got mine close with GPS. I noticed that the tire size and factory setting was different too. The Smarty setting is less than the tire manufacturer size too. If I set it to the tire size using the formulas, then it was still off, so I had to play with the setting and GPS a little.

 

By the way. Using math, the overal diameter of the stock Mich's is 31.6, but my computer was set at 31.4 which was similar to the Mich given diameter.
I guess it's the old "Your results may vary" disclaimer.

 

I marked the tire and the ground. Then rolled 1 revolution and measured. Divide by pi and you're there!