Engine RPM Calculator

Engine RPM Calculator

Last Updated August 16, 2019 | Chris Cervenka

Finding the winning combination for your rear gear ratio can be difficult and require a lot of homework. Nobody likes homework, but when you open the door, start your Mustang up, dump the clutch and feel the quicker acceleration pinning you to the back of your seat… that’s when you know it is all worth it!

The purpose of this calculator is to help those Mustang owners that are contemplating changing out their rear gears. In most cases, you’ll be looking for a more aggressive setup. However, if you go too aggressive, you could be sacrificing serious fuel economy on the highway. This is because you’re making your gears shorter, accelerating through each gear quicker, thus improving acceleration times like 0-60 and 1/4 mile. However, that means in your top gear, or overdrive, you’ll be sacrificing some RPM on the highway for quicker acceleration.

Use this calculator to crunch the numbers and figure out which rear gears would be best for your Mustang’s rear axle. Some Mustang owners may be more concerned with gas mileage, resulting in something like 3.15’s, whereas other owners may want quicker acceleration above all, which could mean 4.10s.

Fill out the info and click calculate!


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An example of this tool in action would be if you drove a 2007 Mustang GT with the 5-speed TR3650 Manual Transmission. Let’s say your Mustang came with 3.31 rear gears and it’s just not aggressive enough for you. If you have the stock 245/45-19 tires, you’ll be looking at 1,912 RPM on the highway at 70 mph. That’s not terribly bad and it allows you to get some pretty good fuel economy.

However, the car feels like a dog getting off the line. A friend of yours suggested to jump to 4.10 rear gears to help with acceleration. This calculator will help you determine that with 4.10 gears, your engine will be spinning at 2,368 RPM at 70 mph. This equates to slightly less fuel economy and slightly more strain on your engine when driving down the highway. Now that you know your RPMs will only increase roughly 456 on the highway, you can confidently make that jump to 4.10 rear gears in your ‘06 GT to better conquer that 0-60 and 1/4 time!

The Equations

If you’re looking to crunch the numbers yourself, it’s truly not that difficult! In order to figure out your Engine RPM at a particular speed, your transmission gear ratio, rear gear ratio and tire size need to be taken into account as all three of those numbers could have an effect on RPM.

If you’re looking to figure out your tire diameter, head over to our Tire Size Calculator for more info!

EngineRPM = (RearAxleRatio x Vehicle Speed x TransRatio x 336.13) / TireDiameter

Engine RPM Calculator

Find out which rear gear ratio is best for your Mustang using the Engine RPM Calculator right here in the CJ’s Resource Center. Whether you’re looking for new rear gears that will provide better gas mileage or quicker acceleration, this Engine RPM Calculator will help you maximize efficiency and acceleration to make sure you get the right rear gears the first time!