The early David Bradley garden tractors came equipped with 1-3/4 and 2-1/4 HP Briggs and Stratton or Continental 4 cycle air cooled gasoline motors .
At the governed engine speed of 3200 R.P.M . the ground travel speed with 16 inch wheels could be changed from 2.1 to 2.4 M.P.H by reversing the double v-belt pulley on the motor.
Later tractors came with 5 or 6 HP motors with 3600 rpm governed engine speeds, and on some models a variable speed changing pulley.
MPH SPEED CALCULATIONS
R x ( MP / CP) x WD x .00011
R = engine rpm
MP = drive(motor) pulley diameter
CP = driven(clutch) pulley diameter
WD = wheel (tire) diameter
.00011 = transmission ratio 27:1 (.037) x PI (3.14) x (12 in/ft) x (5280 ft/mi) x (60min/hr)