Transmission Gear Ratio Equation and Calculator

Mechanics and Machines Calculations Menu

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)


d = distance
ct = Circumference of tire
grt = Gear ratio transmission
grd = Gear ratio differential
ve = Engine speed
vc = Vehicle speed

Tire size calculated

Diameter = [ Width · Profile / 100 · 2 ] / 25.4 + Wheel size ] · 25.4
Note: The height of the tire represents 1/2 of the tire. The tire is a donut shape, thus there are two Rim to outer thicknesses calculated from the % of width profile.


Diameter = Outside diameter of tire mounted on wheel, mm;

Width = Tire thread width, mm;

Wheel size = Also called "Rim" - this is what the tire mounts to, in;

25.4 mm = approximate conversion to 1.0 inches

Profile Height = width x ( % / 100 ) - which is one half the of the tire from where it mounts on the Rim to the outer diameter

Transmission Gear Ratio Calculator
Data for 2004, Chevrolet Corvette C5 Z06 with six speed manual transmission
Width (mm) = Enter the tire width size used
Profile (%) = Enter the tire profile size used
Wheel (Rim) (inch) = Enter the wheel size used
Diameter (mm) = The tire diameter calculated (mm).
Axle Ratio (Final Gear) = Enter the Final Gear Ratio used
  Gear 1ST 2ND 3RD 4TH 5TH 6TH
Gear Ratio
  Enter the gear ratios above      
Calculated Road Speed in Each Gear for Engine RPM
vs Speed kilometers/hour km/h
  1ST 2ND 3RD 4TH 5TH 6TH
Engine Speed
Calculated Road Speed in Each Gear for Engine RPM
vs Speed miles/hour mph
Engine Speed   1ST 2ND 3RD 4TH 5TH 6TH