Armed with three pieces of information, you can calculate the theoretical top speed of your vehicle, as well as the car's speed at any given engine speed. You only need to know the engine speed in revolutions per minute (RPM), the final gear ratio and the outside diameter of the tires. To determine the final gear ratio, it's also important to know the transmission gear ratio as well as that of the differential gear set.
Measure the outside circumference of a tire on the drive axle using a tape measure. In this example, say the tire circumference is 60 inches. A simple way to make this measurement is to place a mark on the bottom of a tire and the ground. Roll the car ahead until the mark on the tire is again on the bottom. Make another mark on the ground at this point, and measure between the two marks on the ground.
Divide the engine RPM by the final gear ratio. For example, if the engine is running at 5,000 RPM and the final gear ratio is 5:1, the result of this math is 1,000 RPM. This is the speed at which the wheel turns.
Multiply the tire circumference by the wheel speed. For our example this math is 60 inches * 1,000 = 60,000 inches.
Divide the result by 12. 60,000 divided by 12 equals 5,000. This is the number of feet travelled in one minute.
Multiply by 60 to determine the feet travelled per hour. 60 * 5,000 = 300,000.
Divide the feet travelled per hour by 5,280 to determine the miles per hour travelled. 300,000/5,280 = 56.8 miles per hour (mph).