How do you calculate RPM at a given speed?

With speed in mph and tire diameter in inches: RPM = (speed × overall gear ratio × 336) / tire diameter. Overall gear ratio = transmission gear × axle (final drive) ratio. Convert km/h to mph and mm to inches before using the same constant.

What is overall gear ratio?

Overall = transmission ratio × axle ratio. In 1:1 direct drive with 3.73 gears, overall is 3.73. In first gear the transmission multiplies that further, so engine RPM climbs for the same road speed.

Why does a taller tire lower cruise RPM?

A larger overall diameter covers more ground per wheel revolution, so the engine turns fewer times at the same road speed (assuming gearing stays the same). Shorter tires raise RPM.

Where do I get tire diameter and gear ratios?

Overall diameter comes from a tire size calculator, manufacturer data, or measurement. Axle ratio is stamped on the diff tag or door sticker; transmission ratios are in the gearbox spec sheet. Common presets (direct, overdrive) are starting points only.

Is this the same math as a gear ratio calculator?

Yes — the inverse of mph = (RPM × tire dia) / (overall × 336). Use this page for a single speed/gear check with a tach view; use the gear ratio calculator when you want a full gear ladder.

RPM at Speed Calculator

Estimate engine RPM at a given road speed from tire diameter, axle ratio, and transmission gear — with a tach view and redline marker.

Engine RPM from road speed, tire diameter, and overall gearing.

Speed & tire

Road speed (mph)

Tire diameter (in)

Redline (RPM)Optional marker on the tach

Gearing

Overall = transmission × axle. Presets are typical street ratios — override with your gearbox sheet.

Transmission gear presets

Axle ratioFinal drive / ring & pinion

Transmission ratioGear you are in (1.0 = direct)

Instrument cluster · tachometer

Engine RPM at road speed

Below redline

Engine RPM Redline 6,500

Engine speed

3,311

RPM @ 70 mph

Overall
3.73
Axle
3.73
Trans
1

51% of 6,500 redline · 26.5" tire

Engine RPM at speed from tires and gearing

Steeper axle gears and lower transmission gears raise RPM at the same road speed.
Taller tires drop cruise RPM; shorter tires raise it — useful after a plus-size or A/T swap.
Compare the result to redline so highway gearing stays comfortable and out of the danger zone.

Whether you are swapping a ring-and-pinion, choosing an overdrive, or upsizing tires, the question is the same: what will the engine turn at highway speed? This free RPM at speed calculator uses the classic hot-rod relationship — RPM = (mph × overall gear ratio × 336) ÷ tire diameter in inches — so you can preview cruise RPM before you buy parts.

Enter road speed, overall tire diameter, axle ratio, and the transmission gear you care about (direct drive is 1.0). The industrial tach shows engine speed against an optional redline marker. Switch to km/h and millimeters when your specs are metric — the tool converts internally to the same mph/inches formula.

Treat the number as geometry, not a dyno. Actual RPM can shift a little with tire growth, load, and converter slip on automatics. Pair this with the gear ratio calculator for a full ladder, and the tire size calculator when you need diameter from a size code.

How to use the RPM at speed calculator

Set road speed and tire diameter. Enter the speed you care about (mph or km/h) and overall tire diameter in inches or millimeters.
Enter axle and transmission ratios. Use your final-drive number and the gear you are in — or tap a common gear preset chip for a starting ratio.
Read engine RPM on the tach. Compare cruise RPM to your redline marker so highway gearing and tire swaps stay in a safe, comfortable band.

RPM at Speed Calculator

Engine RPM at speed from tires and gearing

How to use the RPM at speed calculator

RPM at speed FAQ

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