The following equations and calculators will estimate the resultant vehicle speed at a known engine rpm for a manual transmission with up to six forward gears.
To determine the distance the vehicle will travel for one revolution of the engine by dividing the circumference of the tire by the combined gear ratio of the transmission and differential.
d = ct / (grt x grd)
To determine a vehicles speed from the engine speed by multiplying the circumference of the tire by the engine speed and dividing by the combined gear ratio.
vc = (ct x ve) / (grt x grd)
Where:
d = distance
ct = Circumference of tire
grt = Gear ratio transmission
grd = Gear ratio differential
ve = Engine speed
vc = Vehicle speed
