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.

[Jackpine]

Originally Posted by 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.

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

[stryker6040]

Originally Posted by JackandJanet

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

Thanks for all your help i apperciate it. i plan on calling them on tuesday to see what they recommend i did put more air in them just to be safe. the tire says 50psi max so i put in 45 and it seems to ride better and like i said ill find out for sure on tuesday

[Jackpine]

45 sounds pretty good to me, with 50 psi max. I think you're going to get better handling, less rolling resistance, less heat buildup, better tire wear, etc. etc., at that pressure.

- Jack

[rednoved]

When we have a custom tune made for us, is OUR tire size part of that tune? If it is, can we change it if I were to go to a different size tire? I plan on lifting the truck not too far down the road. Or even if I add a leveling kit in the next month, I'll probably buy larger tires.

[Jackpine]

Originally Posted by rednoved

When we have a custom tune made for us, is OUR tire size part of that tune? If it is, can we change it if I were to go to a different size tire? I plan on lifting the truck not too far down the road. Or even if I add a leveling kit in the next month, I'll probably buy larger tires.

Yes to both questions. You give Bill your tire size and he programs it into the tune. If you change tires later, you can revise the size using the Custom Options menu.

- Jack

[rednoved]

Originally Posted by JackandJanet

Yes to both questions. You give Bill your tire size and he programs it into the tune. If you change tires later, you can revise the size using the Custom Options menu.

- Jack

Thanks. So I wouldn't need to order another custom tune after the lift. That's good to hear!