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Automotive

RPM at Speed Calculator

Engine rpm at any road speed from tires and gearing.

Your measurementsEvery assumption editable01

Top gear is often an overdrive below 1.00.

Engine speedCalculated live02

Result

2162rpm

2162 rpm at 70 mph

Road speed
70 mph
Tire diameter710 revolutions per mile
28.4 in
Axle ratio
3.73:1
Transmission ratio
0.7:1
Engine speed
2162 rpm

The 336 constant folds together inches per mile and minutes per hour, which is why the formula looks arbitrary. It is just unit conversion.

Torque converter slip is not modelled. An automatic with the converter unlocked will turn a few hundred rpm above this figure; locked up, it will sit very close to it.

Use this to check a gearing change before committing. Cruising rpm that lands outside the engine's efficient band costs fuel whichever direction it misses by.

Enter road speed, tire diameter, axle ratio and transmission ratio. This is the calculation to run before committing to a gearing or tire change — cruising revs outside the engine's efficient band cost fuel in either direction.

Why use this tool?What it does differently03

Why use this tool?

The whole drivetrain

Tire diameter, axle ratio and transmission ratio combine into one relationship. Change any and the others must be reconsidered.

Revolutions per mile

Shown alongside, which is the figure speedometer calibration actually works from.

Check before you commit

Model a re-gear or a tire change before spending money on either.

Converter slip flagged

An automatic with the converter unlocked sits a few hundred rpm above this figure.

How this worksThe method04

How this rpm at speed calculator works

Engine speed is road speed multiplied by axle ratio, transmission ratio and a constant of 336, divided by tire diameter in inches. The 336 folds together inches per mile and minutes per hour, which is why it looks arbitrary — it is pure unit conversion.

Revolutions per mile follows from tire circumference: 63,360 inches in a mile divided by the tire's circumference. That figure is what a speedometer calibration module is actually programd with.

Torque converter slip is not modelled. An automatic with the converter unlocked turns a few hundred rpm above this; locked up, it sits very close to it.

How to use itStep by step05

How to use it

  1. Step 1: Enter road speed

    The cruising speed you actually care about, usually motorway pace.

  2. Step 2: Enter tire diameter

    Overall diameter in inches — use the tire size calculator if you only have the size code.

  3. Step 3: Enter both ratios

    Axle ratio from the differential tag, transmission ratio for the gear in question.

  4. Step 4: Compare against the powerband

    Cruising outside the engine's efficient range costs fuel whichever way it misses.

Example usageWorked figures06

Example usage

Highway cruising
70 mph on 28.4 in tires with 3.73 gears and a 0.70 overdrive gives 2,162 rpm — comfortable long-legged cruising.
After a re-gear
The same vehicle re-geared to 4.10 turns 2,377 rpm at the same speed. Better acceleration, measurably worse economy.
After a tire increase
Going from 28.4 to 32 in tires on the original 3.73 gears drops cruising revs to 1,919 — which sounds efficient but often puts the engine below its torque band, causing constant downshifting.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I calculate engine rpm at a given speed?

Multiply speed in mph by the axle ratio, the transmission ratio and 336, then divide by tire diameter in inches. The 336 is a unit conversion constant.

What is a good cruising rpm?

It depends entirely on the engine. Most modern petrol engines are efficient between about 1,800 and 2,500 rpm at cruise; diesels are usually lower. Very low revs in too tall a gear can cost economy rather than save it.

Does this account for torque converter slip?

No. An automatic with the converter unlocked runs a few hundred rpm above this figure. Locked up, it will be very close.

How do I find my tire diameter?

Use the tire size calculator with your size code, or measure the mounted tire's height directly.

Why did my truck get worse after bigger tires?

Taller tires act like a numerically lower gear ratio. The engine drops below its efficient band at cruise, so it downshifts constantly and economy falls rather than improving.

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