Scale speed is if you shrunk yourself down to the size of the the race vehicle, it would be how fast it seems that you are going.
The formula for determining scale speed is:
Scale Speed (miles/hr) = (TrackLength * 3600 secs/hr * VehicleScale) / (5280 ft/mile * Time)
Track Length (in feet) and Vehicle Scale are what you have entered in the Software Settings screen. Time is the racer's elapsed time (in seconds).
If you prefer to use actual speed, you would set the Vehicle Scale to be 1 inch = 1 inch.
Note: Scale Speed is only a calculation. It is not used for determining the standings. It is just for the Ohhh and Ahhh factor.