Visual RPM, Speed, and Gear Ratio Calculator

This program allows you to interactively change the variables that determine your engine's RPM at any given MPH between 0 and 250. You can alter the axle gear ratio, transmission gear ratio, tire height or profile, torque converter efficiency, and see the impact in real-time.

For example, you may wish to know what effect that 4.10 ring and pinion will have on highway cruising speed before you buy it. Or, maybe your speedo is inaccurate and you'd like to know how fast you're really going when the tachometer is at 3,000.

All the transmission and axle ratio fields are dynamic so you can customize them for your car. Drag the floating panels around if they get in your way.

This page is available as a non-Flash page. It's pretty much the same but it's less nice.

  1. Consult the numbers above the gauges or the readout in the 'your setup' panel for the actual RPM and MPH. The gauges show approximations.
  2. The torque converter efficiency calculation is a bit rudimentary. The revs are adjusted up or down based on the percentage set with the slider control.
  3. The default transmission and axle gear set ratios are for a 1968 Mopar B-body, e.g., Dodge Charger, with an A833 4 speed or 727 Torqueflite automatic, and an 8 3/4 axle. The tachometer shown is the famous Tic-Toc-Tach, also from a 1968 Mopar.
  4. The 'estimated 1/4 mile' panel estimates HP using weight and MPH, then estimates an elapsed time. Decreasing the weight will not lower the ET; it will decrease the horsepower variable used to calculate the ET and you'll get the same number. Have a look at my 1/4 Mile ET, HP, and MPH Visual Calculator for a much more detailed look into the subject.
  5. The 'Tire Size' thing is now has it's very own page. Please visit it - Tire Size Visual Calculator
  6. Cubic Inch Displacement Calculator
  7. Altitude Correction Calculator

The formula used for determining RPM given MPH, axle gear ratio, transmission gear ratio, and tire size, if you want to check my math, is the following:

mph  X  336.1524  X  final drive ratio  ÷  tire diameter

Or, the un-simplified long version:

mph  X  12 (inches per foot)  X  5280 (feet per mile)  X  axle gear ratio  X  transmission gear ratio
pi (3.14159265)  X  tire diameter  X  60 (minutes per hour)


Source code for this project is available for
download under the terms and conditions of the
GNU General Public License.