tire size?
 

Hey I know you guys get this question alot but i just want to make sure i get the right number. Whats the correct number for 35x12.5x18? I'm not sure about the squish and that stuff. Thanks guys

OK, A 35 inch diameter tire is 889mm in diameter: 35 x 25.4 = 889

Multiply that by pi: 889 x 3.141593 = 2792.9mm and that is the unloaded circumference.

Assume about 3% reduction for "squish": 2792.9 x 0.03 = 83.8mm we need to subtract.

So, 2792.9 - 83.8 = 2709mm. This is the rounded figure for Tire Size that I'd start out with. Adjust from there.

- Jack

As Jack said... technically a 35" tire would be 2792mm. Compensating for squish, we have found that 2762 works really well for most 35" tires. Of course, the tire manufacturer's specs are the best way to go. If they have a rev/mile value for your tire, that that number and use this formula...

1609344 / Rev per Mile = Circumference in mm

Keep in mind that air pressure, tread wear, and ambient temperature can affect tire circumference and revs per mile. If all else fails, GPS it and make minor adjustments until it's right. :)

thanks guys

Bill always gives a smaller correction for squish than I do when we're talking bigger tires. Either one of our numbers is just a starting point though. If your speedometer is reading too high, your tire size is too large - reduce it. If the speedometer is reading too low, increase the tire size.

- Jack

Guys .. am planning to go for bigger tires .. am running the 35`s now and am planning to run either 37 or 38 .. so what do i have to do with the custom tunes .. it will take long time until i regear it .. thanx

You don't really have to do anything except change the Tire Size in the Custom Options menu (if you want your speedometer to read correctly). Your truck is going to feel underpowered though.

- Jack

Going with 38's, I'd recommend 4.56 gears. In fact, 4.56 with 38's would give you the same RPM/MPH as 3.73 gears with the stock 31's. Until you can regear though, the truck is going to feel like like an absolute slug.

In the mean time, you can adjust the tire size and gear ratio using the custom options.

Take care.