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A couple of potential sources of error

Tire expansion

There is a potential source of error in these calculations due to centrifugal force expansion of the tire at high speeds. While that effect may often be negligible for radial-ply tires due to the circumferential belts used in their construction, to get the most accurate results (especially using bias-ply tires) users should check with their tire manufacturer to determine how much the tire radius (or revolutions per mile) will change at various speeds.

RPM delivered to the transmission

Another potential error source in these calculations is when the engine RPM is not identical to the RPM of the transmission input gears. For example, using this calculator for a vehicle having a torque converter (such as an automatic transmission), or using this calculator for a vehicle with a slipping clutch may produce incorrect results. In such cases, the engine RPM may be significantly higher than the RPM delivered to the transmission gears, and the calculated vehicle speed will then be significantly higher than the actual speed. On the other hand, if the torque converted is fully locked, or if the clutch is not slipping, these calculations should be reliable.