What is RPM?

Most of us are familiar with the term RPM (revolutions per minute), often seen on a car’s dashboard tachometer. But what exactly is RPM and why is it crucial in the automotive world? RPM refers to the rotational speed of an object, commonly used to describe an engine’s speed. In simple terms, it signifies how fast the engine’s crankshaft rotates in an internal combustion engine or how an electric motor’s rotor spins.

Engine Rotation Basics

In internal combustion engines, the fuel-air mixture ignites in cylinders, pushing pistons connected to a crankshaft. This transforms linear piston movement into the rotation of the crankshaft – the essence of engine RPM. Electric motors, on the other hand, utilize magnetic fields and currents to induce rotor rotation.

Calculating RPM

To understand RPM, we must delve into the vehicle’s powertrain. From the engine, the rotational force travels through the drivetrain, consisting of gears and mechanical links. The transmission ratio, a critical factor, defines the relationship between the engine RPM and the rotation speed of the wheels. This connection is expressed by the equation: RPM = Wheels RPM × Transmission Ratio.

Tire Size Matters

Beyond the drivetrain, the car’s movement transforms from rotation to linear motion through the tires. The tire diameter, a key factor influenced by size, determines the final transformation ratio. This relationship is captured by the equation: Vehicle Speed = Wheels RPM × Tire Diameter × π × 60 / 63360.

Using the RPM Calculator

The RPM calculator serves as a versatile tool to convert between RPM, vehicle speed, transmission ratio, and tire diameter. For instance, if you know your speed, gear, and RPM, the calculator can unveil the transmission ratio and wheel RPM. Similarly, if contemplating new wheels, inputting tire diameter, speed, and transmission ratio can reveal the corresponding RPM.


Let’s say you’re driving at 60 mph in 3rd gear with a tachometer reading of 3,500 RPM. Input your tire diameter (calculated using a tire size chart) and RPM into the calculator. The result provides your drivetrain transmission ratio and wheel RPM.


1. What does RPM stand for? RPM stands for revolutions per minute, denoted as min⁻¹.

2. How do you calculate revolutions per minute? To calculate RPM, multiply wheels RPM by the transmission ratio.

3. How do I convert vehicle speed into a wheel’s RPM? Multiply the wheel’s RPM by the tire diameter, then by 60 and π, finally dividing by 63360.

4. What are the units for RPM? RPM is measured in min⁻¹.

This comprehensive guide demystifies RPM, explaining its significance and providing practical insights into its calculation and application in understanding a vehicle’s speed.

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