Here's two formulas to help determine your current RPM's and MPH. You can also check your speedometer for correct calibration by plugging your RPM's for a given speed into the formula.
The formula can then be used to determine what the change in tire size will do to your RPM's.
MPH X DRIVE RATIO X 336
---------------------------------------- = RPM'S
If you would like to know your speed at a particular
RPM, the formula is:
RPM X TIRE DIAMETER
--------------------------------------- = MPH
DRIVE RATIO X 336
(In both cases, "336" is simply a constant to convert the tire diameter to the linear distance the tire travels)
Determining your Drive Ratio
The drive ratio may be located at the following locations depending on how old of a vehicle it is:
If nothing is listed for the rear end then you can use the following method to determine the gear ratio:
- A newer vehicle will often times have it listed on a spec sheet glued to the glove box door
- An older vehicle can have a tag located on the differential cover. On 4x4 it could be on the front axle.
- Their are also stamping codes located on the passenger axle that can provide differential information.
If for example, the drive shaft turns just a little over three turns the you probably have a 3:08 gear ratio.
- Raise the rear end and place it on jack stands
- place a chalk mark on the tire and the drive shaft
- have someone count the revolutions of the drive shaft as you rotate the tire one complete turn
- This will give you a close approximation of the gear ratio
All this may seem like a lot of work but knowing what the effect of a tire change will have on your engines working rpm can save it from excessive wear and tear and maybe even extend its life.
Here's a little quick reference table to show how the rpm's change with tire size. At 60MPH.