To expand on Les' post and assuming a manual gearbox:
Engine RPMs
times transmission gear(s) ratio
times (if used) differential ratio = effective Driven Wheel(s) RPM (DWRPM). DWRPM
times wheel circumference
times 60 = MPH or KPH (depending on your choice of English vs. metric system).
For example (English):
Engine RPMs = 2,000
Transmission Ratio (2nd gear) = 2:1 (0.5)
Differential Ratio = 3.11:1 (0.322)
Driven Wheel diameter = 26" (81.68" circumference, or 6.8 feet)
2,000 (0.5 X 0.322) = 322 DWRPM.
DWRPM X 6.8' = distance driven of 2,189.6' in 1 minute, or 131,376' in 1 hour, or 24.9 MPH. If the transmission were in a 4th gear (with a 1:1 ratio), then the car would be traveling 49.8 MPH.
If the vehicle has an automatic gearbox, the associated torque converter (used instead of a clutch) will make the calculation impossible.
True enough, but if it is assumed that the lock-up clutch is engaged, the math holds. And if not, the particular torque converter speed ratio values can be applied to give "close enough" DWRPMs.