Speed from RPM Calculator

Engine speed, rpm

Gear ratio

Final drive

Tyre width, mm

Profile, %

Rim, inches

Result

Speed, km/h88.9
Revs at 100 km/h3,376
Wheel speed, rpm769.2
Rolling circumference, m1.926
Total ratio3.9
Wheel diameter, mm632

From crankshaft to contact patch

Works out road speed from engine revs, gear ratio and final drive, and gives the revs you will see at 100 km/h.

The chain is short: engine revs divided by the gear ratio and the final drive give wheel revs. Multiply those by the rolling circumference of the tyre and you have road speed.

Rolling circumference is about three percent short of the geometric one, because a loaded tyre squats and the radius to the road shrinks. The calculation includes that correction; without it the numbers drift a couple of km/h.

What this calculator shows

Road speed at the revs and gear you enter
The revs you will see at 100 km/h — the classic measure of motorway comfort
The rolling circumference and the total ratio through the drivetrain

What to keep in mind

Rolling circumference is taken at three percent below geometric, the usual deflection under the weight of a car.
Torque-converter slip is ignored: with an unlocked automatic, real speed will be slightly lower.

FAQs

What revs should I see at 100 km/h?

On the top gear of most modern cars, 1800 to 2500 rpm. Over three thousand points to a short final drive or too few gears.

How does the final drive affect performance?

A short final drive with a high ratio gives acceleration but raises motorway revs and fuel use. A long one does the opposite: relaxed on the motorway, sluggish in town.

What is rolling circumference?

The distance a wheel travels in one revolution. It is shorter than the geometric circumference because a loaded tyre deflects, cutting the effective radius by roughly three percent.

Why did the speedometer change after new tyres?

Because it counts wheel revolutions against the factory rolling circumference. A different overall diameter means a different real speed at the same rev count.

Worked example

Direct drive on the motorway

Input: 3000 rpm, ratio 1.00, final drive 3.9, 205/55 R16

Output: 88.9 km/h, and 100 km/h means 3376 rpm

Note: Over three thousand revs at a hundred is noisy and thirsty. That is exactly why modern cars grew sixth and seventh gears at around 0.7 — they drop motorway revs to about two thousand.

Useful links

All tools in Cars

Speed from RPM Calculator

Result

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