Max, I agree with you 100% that 3-shaft/4-shaft is a good quick way to identify different gearboxes. But to figure out the exact top speed you will get, requires knowing the motor RPM, the actual gearbox ratio and the diameter of the drive wheel.

The simplified calculation, including the 1/16 scale speed factor, etc etc is:

Scale speed, in scale MPH = ( Motor RPM / gearbox ratio) x drive wheel diameter in inches x 0.05

Example for a Tamiya KT:
Scale speed, MPH = (16,600 motor RPM / 60 gear ratio) x 2-inch wheel diameter x 0.05 = 27 scale MPH

The actual top speed will be a bit slower - maybe 4 to 6 mph slower - because the motor under load will be slower than the measured no-load speed of 16,600RPM. Also, fully charged batts (about 8 to 8.4V) will kick up the speed over the 7.2V nominal voltage.