RPM Calculator (engine rpm from speed, gear ratio, final drive & tyre size)

Understanding the engine RPM calculator — concept and principle

This calculator works out engine speed from road speed, the selected gear ratio, the final drive ratio and the tyre size, and gives the wheel rpm, tyre diameter and circumference, speed at 1,000 rpm and the overall ratio. Drivers, tuners and students use it to check cruising rpm or the effect of a tyre or gearing change.

Road speed sets wheel rpm; gearbox and final drive multiply it up to the crankshaft.

How it is calculated

Tyre diameter (mm) = rim × 25.4 + 2 × width × aspect ÷ 100

Circumference = π × diameter

Wheel rpm = road speed (m/min) ÷ circumference

Engine rpm = wheel rpm × gear ratio × final drive

Tyre size as in 205/55R16: width in mm, aspect ratio in %, rim diameter in inches.

The principle

Each turn of a wheel moves the car one tyre circumference. Dividing speed by circumference gives how fast the wheels turn. Between the engine and the wheels, the gearbox and final drive multiply the turning speed: the engine turns gear ratio × final drive times for each wheel revolution. A taller tyre or a lower (numerically smaller) ratio means fewer engine revolutions for the same road speed.

Worked example

Default inputs: 100 km/h, gear 0.82, final drive 4.1, tyre 205/55R16.

Diameter = 16 × 25.4 + 2 × 205 × 0.55 = 406.4 + 225.5 = 631.9 mm; circumference = 1.985 m.
Wheel rpm = (100,000 ÷ 60) ÷ 1.985 = 840 rpm.
Overall ratio = 0.82 × 4.1 = 3.362.
Engine rpm = 840 × 3.362 = 2,823 rpm; speed at 1,000 rpm = 35.4 km/h.

Each point is the calculator re-run with only “Road speed” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Road speed	Engine speed (rpm)
25	706
50	1,411
75	2,117
100	2,823
125	3,528
150	4,234
200	5,645

How to read the result

Real rpm can differ by a few percent because the loaded rolling radius is a little smaller than the free diameter and tyres grow slightly at speed. With an automatic gearbox, the torque converter may slip unless locked up. A larger tyre lowers rpm and makes the speedometer read low.

Common mistakes

Mixing up width and aspect ratio in the tyre code.
Forgetting the final drive ratio.
Using the overall ratio of a different gear.

Keep in mind: Changing tyre size affects speedometer accuracy and may be subject to regulations. Check with the vehicle maker.

Frequently asked questions

How do I find my tyre diameter?

Rim in mm plus twice the sidewall height, where sidewall = width × aspect ÷ 100.

Why does my car show slightly different rpm?

The loaded tyre radius is smaller, and automatic gearboxes may slip.

What is the overall ratio?

Gear ratio times final drive ratio.

Will bigger tyres lower rpm?

Yes, in proportion to the increase in circumference.

Sources: ISO 4000-1 / ETRTO metric tyre size designation; standard gear kinematics

Checked 9 Nov 2026. Metric tyre size designation of ISO 4000-1 / ETRTO (section width mm / aspect ratio % / rim diameter code in inches); gear kinematics n_engine = n_wheel × i_gear × i_final (standard formula). 1 mile = 1,609.344 m.

Your numbers