Wheel Size, Rear Gear Ratio, RPM, Speed Calculator.
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Drag the floating panels around if they get in your way.
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 input fields are dynamic so you can customize them for your car.
Thank you to Vexer.com for the calculator.
Consult the numbers above the gauges or the readout in the ‘your setup’ panel for the actual RPM and MPH. The gauges show approximations. The revs are adjusted up or down based on the percentage set with the slider control. The default transmission and axle gear set ratios are common for Mopar vehicles from the late 60’s / early 70’s 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. 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.
The formula used for determining RPM given MPH, axle gear ratio, transmission gear ratio, and tire size, if you want to check the math, is the following:
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)