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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.

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)