Drivetrain Calculators

The drivetrain turns engine rpm into road speed, and the numbers that link them are the axle ratio, the transmission gear, and the tire diameter. Change any of them and the speedometer, the engine rpm at cruising speed and the acceleration all change together.

Which tool answers which question

• Drag racing gear selector: The axle ratio that puts your engine where you want it as you cross the finish line. Use a real trap speed if you have one, or estimate it from horsepower...
• Driveshaft critical speed calculator: Every driveshaft has a speed where it starts to whip and vibrate. This finds it from your tube size and length, and translates it into the top speed your...
• Gear ratio, rpm & speedometer calculator: Cruise rpm for your axle ratio and tires, and what happens when you change tire size: how far off your speedometer will read and what ratio gets your...
• Quarter-mile ET and MPH calculator: Predict what your car should run from its weight and horsepower, or work backward from a timeslip to estimate how much power it's making.
• Speedometer drive gear calculator: For cable-driven speedometers: the driven gear tooth count that makes your speedometer read true after a gear or tire change, and how far off the nearest...

Gear ratios, rpm and the speedometer

Engine rpm at a given speed depends on the tire diameter, the axle ratio and the transmission gear. A numerically higher axle ratio accelerates harder and turns more rpm at the same speed. If you change the axle ratio or the tire size, the speedometer reads wrong unless you correct it, and the speedometer gear tool works out the right driven gear.

Driveshaft speed and drag strip estimates

Every driveshaft has a critical speed at which it begins to whip, set by its length, diameter and wall thickness. Builders keep the working speed well below that, and a common practice is to stay under about 75 to 80 percent of it. The drag strip estimators turn weight and power into an elapsed time and trap speed, and the result depends on traction, gearing and conditions, so treat it as a ballpark.

Common mistakes

• Forgetting the tire diameter. A taller tire acts like a numerically lower gear.
• Trusting a stock speedometer after a gear change. Correct it or check with GPS.
• Ignoring driveshaft length and speed. A shaft that is too long or too fast can fail dangerously.
• Reading an estimate as a promise. Real times depend on traction and tune.

Guides for this section

Want the background behind these tools? These guides explain the thinking and the numbers in more depth.

• Gear Ratios Explained: Axle, Transmission and Tire Size
• Towing Basics: Hitch Classes, Tongue Weight and Ratings
• Reading a Tire Sidewall and Choosing a New Size
• All guides