Gear Ratio Formulas
To figure out what RPM your engine will be turning with any sizetire and gear ratio you will need to use the following formula.
MPH x Gear Ratio
X
336 = RPM
Tire Diameter
If you know the tire size and rpm at a certain speed then you can figure out what gear ratio you have in your car by using this next formula
RPM x Tire Diameter
Divided by 336 = Gear Ratio
MPH
To keep the same overall gear ratio when you change the size of the tires on your vehical you can use this formula
New Diameter x Old Gear Ratio
= New Gear Ratio
Old Diameter