Engine RPM
rpm
Drivetrain transmission ratio
Wheel RPM
rpm
Tire diameter
in
Vehicle speed
mph

RPM Calculator

By Salam Moubarak, PhD
Last updated: Jan 11, 2021

This RPM calculator helps you understand what is RPM and shows how to compute the relationship between the speed of a vehicle and the RPM of its engine. You can also use it as an RPM to mph calculator.

The first section explains the RPM definition and its extensive use in the automotive industry. In the second part, you will see how engine RPM relates to vehicle speed, and understand the effects of drivetrain transmission and tire size. Finally, we will show you, with an example, how to use our RPM calculator.

What is RPM?

We've all heard the term "RPM" and seen an RPM gauge (tachometer) in a car's dashboard. We've also probably noticed that engine noise goes up with RPM. But what exactly is RPM? Why is it so extensively used when we talk about cars and engines?

The RPM term can be employed for any rotating object. It means revolutions per minute or rotations per minute. Technically, RPM is a unit of rotational speed; but in reality, it's more often used to say things like "engine RPM", meaning: "The engine's rotational speed". As if when saying: "What's your car's mph?" and meaning: "What's your car's speed?".

You might be wondering what do we mean by "engine rotation"? What exactly is rotating in the engine? - Because the whole thing definitely isn't!

Well, in very simplified terms, in an internal combustion engine, fuel and air burn in the cylinders and push the pistons. These pistons are linked to what we call a crankshaft through connecting rods. This mechanism transforms the linear movement of the pistons into rotation of the crankshaft. It is this crankshaft rotation that we are referring to when we talk about engine RPM.

In an electric motor, electric currents and magnets generate magnetic fields that interact with the windings of what we call the rotor. A mechanical torque is created; it acts on the rotor and generates its rotation. The speed of the rotor's rotation is the electric motor's RPM.

Now that you understand the RPM definition and how the crankshaft rotation (or the rotor rotation) is created, read on if you want to learn how to calculate RPM, what's a drivetrain transmission ratio, and see the relationship between the wheels' speed and the engine's RPM used in this RPM to mph calculator.

How to calculate RPM?

In the following sections, we will refer to engine rotations per minute as simply "RPM". Any other rotating item will be referred to by name (for example, we will say "wheels RPM" when referring to the wheels' speed in rotations per minute). Before we learn how to calculate RPM, we will see what factors impact an engine's RPM and what moving parts of a car are related to the RPM.

Let's continue our journey through the moving parts of a vehicle's powertrain that started in the engine cylinders: After reaching the crankshaft, the rotational movement continues through the car's drivetrain, mostly composed of mechanical links and gears. These gears, located in the gearbox and final drive, take the RPM as their input speed and transform it to a different speed at their output shaft, linked to the driving wheels. Therefore, this output speed is the wheels' speed and the ratio between the wheels' speed and the RPM is called the "transmission ratio".

This relationship between the RPM and the wheels RPM can be expressed by the following equation:

RPM = Wheels RPM * Transmission ratio

Thanks to the gearbox, this transmission ratio can take different values for different driving situations. This is crucial for the proper and efficient functioning of a vehicle's powertrain. But that's another story...

After reaching the driving wheels, the rotational movement of a car's powertrain is transformed into a linear movement of the vehicle itself thanks to the contact between the tires and the road. The ratio of this final transformation depends solely on the tire size, and more specifically, the tire diameter. You can easily find tire diameters in readily available tire size charts.

This relationship between the wheels' RPM and the vehicle speed can be expressed in this second equation:

Vehicle speed = Wheels RPM * Tire diameter * π * 60 / 63360,

where:

  • π is the circle constant; it represents the ratio between the tire's perimeter and its diameter;
  • 60 is the number of minutes (from "rounds per minutes") in an hour (from "miles per hour"); and
  • 63360 is the number of inches (from the Tire diameter) in a mile (from "miles per hour").

Now that we covered the journey between the burning fuel and the moving car, we can see that there's a direct relationship between the RPM and the speed of a vehicle; and this relationship depends on two factors only: The transmission ratio and the tire diameter.

In the following section, we will see how we can use our RPM calculator to quickly and easily convert RPM to mph and vice versa.

How to use the RPM calculator?

In a nutshell, with this RPM calculator, you can compute any one of the RPM, vehicle speed, transmission ratio, or tire diameter by providing the other three!

For example, let's say you are driving a car at 60 mph, you're in 3rd gear, and your tachometer indicates 3,500 rpm. You know that your tire code is: 185/55R15.

  • First, you can look up a tire size chart or, more conveniently, use our tire size calculator to get your tire diameter. In this case, you will get Tire diameter = 23 in; then
  • Input your tire diameter in the field Tire diameter;
  • Provide your RPM in the field RPM; and
  • Enter your speed in the field Vehicle speed.

The RPM calculator will give you your 3rd gear drivetrain transmission ratio in the field Drivetrain transmission ratio (= 3.99). Notice that this is a ratio between two rotational speeds; thus, it does not have a unit. The calculator also gives you the wheels' speed of rotation in the field Wheel rpm (= 877 rpm).

Now let's say that you wish to get a new set of sportier wheels for your car. You're thinking 17-inch rims in 205/45R17, and you want to know what your RPM would be when you're running at 60 mph in 3rd gear?

No problem! All you have to do is:

  • Calculate your new tire diameter using our tire size calculator (in this case, Tire diameter = 24.26 in) and input this new value in the field Tire diameter of the engine RPM calculator;
  • Provide your vehicle speed (= 60 mph) in the field Vehicle speed; and
  • Enter your (previously calculated) transmission ratio (=3.99) in the field Transmission ratio.

The calculator will give you your new RPM at 60 mph in 3rd gear (= 3318 rpm) in the field RPM. Notice that the new Wheel RPM is also lower (= 831 rpm) than the old one (= 877 rpm). That is normal because the new wheels are bigger than the old ones, so the engine and the wheels do not need to turn as fast to produce the same vehicle speed of 60 mph!

Salam Moubarak, PhD
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