Gear Ratio Speed Calculator (MPH)

It is important to have the correct gear ratio for the tire size for proper operation of your clutch or torque converter. A improper gear ratio will make the clutch or belt slip and cause damage to the unit very quickly.

To determine the gear ratio necessary for your go kart or mini bike you must know the limit of your top speed. When using a clutch to drive your go kart the top speed on flat terrain is 35mph, if the terrain is hilly or the weight of the driver is high the top speed should be 25mph. Mini bikes work best with a top speed of 25mph for all terrain.

When using a torque converter with a go kart you may go as high as 45mph on flat terrain and 35mph on hilly terrain. For mini bikes 35mph is the top speed when using a torque converter.

Use our calculator to determine the top speed and gear ratio for your tire size. Input the tire diameter, highest engine rpm, number of teeth on your clutch or torque converter jack shaft and the number of teeth on the axle sprocket. If you go higher than the recommended mph then the possibility of excessive slipping of the clutch or belt may cause early failure. Changing the tire size will dramatically change the top speed so if you are going to a larger tire size you must correct the gear ratio for proper operation. 

Required Data Entry
Unit Number
Drive Wheel Diameter In Inches
Rated Engine RPM
Teeth On Axle
Teeth On Clutch
Calculated Results
Drive Wheel Circumference In Inches
Gear Ratio
Maximum Speed In MPH

This converter requires the use of Javascript enabled and capable browsers.

The purpose of this calculator is to determine the potential speed of a go kart based on certain data.

  1. Enter the diameter of the drive (power) wheel and tire combination. (Make sure the tire is properly inflated and in good condition.)
  2. The diameter is the radius * 2, or in this case, twice the distance from the ground to the center of the axle.
  3. Enter the published maximum rated sustainable RPM for the engine you are using.
  4. Standard "lawnmower" engines might be as few as 2400 and high performance engines as high as 9000 RPM.
  5. The majority are from 3000 to 4000 RPM.
  6. Enter the number of teeth or sprocket splines on the axle (higher number, probably 50 to 70) and on the clutch or engine (smaller number, probably 8 to 15).
  7. The ratio of those numbers yields the best power (faster acceleration) or the best high end speed, or a compromise of both. The higher the gear ratio, the higher the top end speed; the lower the ratio, the stronger the acceleration (power).
  8. Click on Calculate. The results are the circumference of the drive wheel/tire combo (in inches), gear ratio and the potential maximum speed (in MPH).
  9. Our default is for a slow, safe junior kart. The gear ratio is 5 and the potential speed is just under 13 miles per hour.

    Applicable Calculator Formulae

    PI = 3.14159
    PI * D = C (3.14159 * wheel/tire diameter = circumference of wheel and tire combo)
    at / ct = GR (gear ratio = axle teeth / clutch or engine teeth)
    RPM / GR = WRPM (engine RPM / gear ration = Wheel RPM)
    C / 12 = Cf (circumference of wheel/tire combo in feet = circumference / 12 inches)
    FPM = Cf * WRPM (wheel RPM * circumference in feet = feet per minute)
    FPM * 60 = FPH (feet per minute * 60 minutes = feet per hour)
    FPH / 5280 = MPH (feet per hour / 5280 feet = miles per hour)