Tire Size Change Question

[Power Hungry]

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.