[Phil B]

Originally Posted by NeillNZ

Can the tyre shop tell you the specification for the rolling circumference? Failing that, what size are the rear tyres and what is your diff ratio?

I'll have more of a difference than your example, but thanks for that, it really helped me crunch some numbers.

My speedo is pretty much right on now with my 265/50R14s.
My new tyres will be 295/50r15.
Diff ratio is 2.92
Gearbox is a Single Rail

I used an online calculator to work out this:
New tyres will have 174.05mm more circumference than the old.
Old tyres give 1497.7 turns/km out the back of the gearbox.
New tyres give 1375 turns/km out the back of the gearbox.


My question now is: how does this relate to number of teeth on the speedo gear?