How much does tire size change affect ratio?

[KatDiesel]

Originally posted by spitty:
KatDiesel,
The number 4. 10 is the ratio of how many rotations the pinion has to make for 1 rotation of the ring gear. With 4. 10 meaning the pinion will make 4. 10 revolutions to the ring gears 1 revolution. It has nothing to do with the original size tires that came on the truck. If a truck came with 245's and a 4. 10 and you changed to 295's, the final drive ratio will be the same as that of a truck which came with 265's and a 4. 10 and you changed to 295's. This is my understanding of how it works. Please correct me if I am wrong.

The actual ratio will be the same in the differential yes, overall the ratio will change when you change tire size. The ratio will always be 4. 10 (if thats whats on the truck) but after factoring in tire size it is no longer 4. 10.

How can the ratio remain the same if from the factory you had 245's (30. 5 diameter) and switched to 295's (33. 40 diameter) how can the ratio remain the same? The 295's are going to turn less revoltions to go the same distance as the 245's.

If it doesn't change your ratio then why does the RPM's change with a tire size swap?

 

[Papa Joe]

Kat's right on the money, and he explained it well, too.
My assumption is the "computer" is programmed with the gear ratio and the factory tire size when it leaves the plant.
My question is can we re-program the computer when we elect to change tire size so the speedometer will be correct.

------------------
White 2001 QuadCab 2500 Laramie SLT, HO 6-speed, 3. 54 limited slip, Mopar exhaust brake, towing/camper options, Rhino liner, Grover Air Horns (not yet installed), Un BOMBed (so far), DUAL AIR BAGS !!