Thread: Tire Size Change Question
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Old Sat, May 23rd, 2009, 08:29 PM
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Originally Posted by Power Hungry View Post
One thing to keep in mind is that the gear ratio calculator is not always 100% accurate. It is a best guess as to diameter based on the tire size designation. Tires with the same designation can actually vary quite a bit when measured and I've seen differences as much as 5% between different tires of the same "size". Even tires listed as 35" can vary +/- ½".

The first place to look is the tire manufacturer's website and see if they list a "rev per mile" value. If they do, take this value and divide it into 1609344 (ie. 1609344/rev per mile). This will give you the tire size in mm.

The next best thing is to park the truck on a long, flat surface. Mark the ground and the tire where it touches the ground. Roll the tire forward (or backward) 3 rotations and mark the ground again. Measure the distance and divide by 3. This gives a really accurate tire size value. If using inches, remember to convert to mm by multiplying by 25.4.

Hope this helps.
From what I've observed in my own truck and from feedback from several Ford owners on the forums, the 3% reduction in circumference seems a good starting place. Then, it's pretty simple to calculate any remaining percentage error and apply that to the "squished" circumference. It's what I did.

stryker6040 - PLEASE DO find out from Procomp what pressure they recommend. Don't let it go below that value when the tires are cold.

- Jack