Scale speed = measured speed × scale factor. A 1/10 car doing a real 30 mph looks, relative to its size, like a full-size car at 300 mph. It is how fast it appears for its scale.

1/10 buggy/touring, 1/8 buggy, 1/5 large-scale, 1/16 & 1/18 mini, 1/24 micro. Bigger denominator = smaller model = higher scale speed for the same real speed.