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
TIRE DIAMETER
 

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

• 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.

• 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