Can you measure the speed of a car by ratio?

Can you measure the speed of a car by ratio?
Yes. To do so, you will need your transmission gear ratios, your differential gear ratio and your tire circumference. You can derive the tire circumference by mutliplying its diameter by 3.1416 (pi).
Example: At highway speed your RPM is 2000. Say your overdrive (5th gear) is 0.84:1, your differential gear ratio is 3.73:1 and your tire circumference for a P215/75R15 is 87". First divide the RPM by the first ratio (.84) which gives you 2380 RPMS. Then divide by the differential ratio of 3.73 gives you 638.3 RPM (Wheel speed). You speed is 87"x 638.3 = 55,532 inches per minute or 52.6 MPH.

Hope that helps!