RPM to Speed Calculator Widget

A gearing calculator for car and motorbike enthusiasts. Type the engine speed, the gear ratio, the final drive and a tyre size such as 225/45R17 and get the road speed in km/h and mph - or reverse it to see what the engine turns at 70 mph after a final-drive or tyre change.

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<iframe src="https://a2z.tools/embed/w/rpm-speed-calculator" title="RPM to Speed Calculator by A2Z Tools" width="100%" height="620" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

The wheel turns at the engine speed divided by the gear ratio and the final drive ratio. Each wheel revolution moves the car one tyre circumference, pi times the overall diameter, so speed is wheel rpm times circumference times 60 minutes. The diameter comes from the sidewall code: a metric size W/A R D has a rim of D inches plus two sidewalls each W x A/100 mm tall, so 225/45R17 is 431.8 + 202.5 = 634.3 mm. Prefixes like P or LT, speed letters such as ZR and a load index after the size are tolerated, and flotation sizes such as 31x10.50R15 give the diameter directly as 31 inches. You can also type a measured diameter in mm or inches. At 3,000 rpm in a 1.000 gear with a 3.73 final drive on 225/45R17 tyres the car does 96.2 km/h (59.8 mph). The reverse mode solves the same equation for rpm. Tyre squash under load and clutch or torque-converter slip are ignored.

Calculation method

  • Tyre diameter (mm) = rim (in) x 25.4 + 2 x width (mm) x aspect / 100
  • Wheel rpm = engine rpm / (gear ratio x final drive)
  • Speed (km/h) = wheel rpm x pi x diameter (m) x 60 / 1000
  • Engine rpm = speed (km/h) / (pi x diameter x 60 / 1000) x gear x final drive
  • 1 mile = 1.609344 km

Worked examples

Speed in direct drive

Inputs: 3,000 rpm; gear 1.000; final drive 3.73; 225/45R17

Result: 96.2 km/h (59.8 mph); wheel at 804 rpm

Diameter 634.3 mm, circumference 1.993 m.

Motorway revs

Inputs: 70 mph; gear 1.000; final drive 3.73; 225/45R17

Result: About 3,514 rpm

70 mph = 112.65 km/h, or 942 wheel rpm.

Limitations

  • Uses the nominal unloaded tyre diameter; a loaded tyre is a little smaller and tyres wear.
  • Ignores clutch and torque-converter slip and wheelspin.
  • Unusual tyre formats (old alpha-numeric sizes, motorcycle inch sizes) need the diameter entered directly.

Where publishers use it

  • A performance-parts shop's ring-and-pinion product pages
  • A 4x4 forum thread on regearing after fitting bigger tyres
  • A track-day blog choosing the right gear for a corner
  • A motorcycle sprocket-change guide
  • An automotive engineering class exercise on drivetrain ratios

Questions

Where do I find the gear and final drive ratios?

In the owner's manual, a service manual or the transmission maker's specification. A direct-drive gear is 1.000 and overdrive gears are below 1, often around 0.7-0.85; car final drives commonly sit between about 3 and 4.5.

How does a bigger tyre change speed?

Speed at a given rpm rises in proportion to the diameter. Moving from 634 mm to 660 mm (about 4% taller) makes the car about 4% faster at the same rpm - and the speedometer reads about 4% low.

Why does my speedometer disagree?

Speedometers are allowed to over-read and most do, and the real loaded tyre is slightly smaller than its nominal size. A GPS speed is the best check against this widget.

Can I calculate rpm at motorway speed?

Yes - choose engine rpm from road speed. With the 1.000 gear, 3.73 final drive and 225/45R17 tyres, 70 mph needs about 3,514 rpm; a taller 0.8 overdrive gear would bring that to about 2,811 rpm.

Does it work for automatic gearboxes?

With the torque converter locked up, yes. Below lock-up the converter slips, so the engine turns faster than this calculation shows.

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools RPM to Speed Calculator
https://a2z.tools/embed/rpm-speed-calculator

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