Gear Speed Calculator

Road speed in every gear from rpm, gearbox ratios, final drive and tyre size, with rpm at cruising speed, rpm drop on each upshift and an rpm versus speed chart.

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About this tool

What is the Gear Speed Calculator?

Engine speed turns into road speed through three numbers: the gearbox ratio, the final drive and the tyre. Change any of them and every gear's top speed, the motorway cruising rpm and the drop after each shift all move. This calculator works out the whole picture at once. Type a tyre size such as 225/45R17, the ratios for each gear, the final drive and the redline, and it lists the speed at redline and at any rpm in each gear, the rpm at your cruising speed, and the engine speed you land on after each upshift.

A chart plots rpm against speed for every gear so you can see the gaps between ratios and where a cruise speed sits on each line. Speeds come out in km/h or mph. If you know the tyre diameter rather than the size code, enter that instead, in inches or millimetres. Flotation sizes such as 33x12.50R15 are understood too.

  • Tyre diameter from metric codes (P, LT and ZR included), flotation sizes or a typed diameter
  • Up to eight gear ratios plus final drive and redline
  • Speed per gear at any rpm and at redline, rpm at a cruise speed, rpm after each upshift
  • rpm versus speed chart for every gear with redline and cruise markers
  • km/h or mph, copy results and share link
Read the guide: Road Speed from RPM, Gear Ratio and Tire Size: The Formula

How to use the Gear Speed Calculator

  1. Choose km/h or mph, then enter the tyre size code or switch to diameter and type it in inches or millimetres.
  2. Enter each gear ratio; add or remove gears to match the gearbox (up to eight).
  3. Enter the final drive (axle or differential) ratio and the redline.
  4. Enter a cruise speed and an engine speed of interest.
  5. Read the table: speed at your rpm and at redline in each gear, rpm at the cruise speed, and rpm after each upshift. The chart shows the same thing graphically.

Worked examples

Six-speed coupe at the motorway

Input

225/45R17, ratios 3.36, 2.07, 1.43, 1.00, 0.84, 0.56, final drive 3.42, cruise 110 km/h

Output

1,762 rpm in 6th, 2,643 rpm in 5th, 3,146 rpm in 4th

The very tall 0.56 sixth gear is an economy gear. Its 405.8 km/h speed at redline is a gearing limit only; drag stops the car far sooner.

Shift points

Input

Same car, redline 6,500 rpm

Output

After each upshift: 4,004, 4,490, 4,545, 5,460 and 4,333 rpm

The 4th-to-5th shift is the closest pair, the 1st-to-2nd the widest. A gap that drops below the engine's torque peak is where acceleration suffers.

Classic US formula check

Input

26 in tyre, 1.00 gear, 3.73 axle, 3,000 rpm

Output

62.21 mph

Matches 3,000 x 26 / (3.73 x 336.13). Swapping to a 4.10 axle with 33 in tyres gives 59.9 mph at 2,500 rpm.

Light truck tyres

Input

LT265/75R16

Output

Diameter 803.9 mm (31.65 in)

The calculator ignores the LT prefix and any load index and speed rating after the size.

Background and insight

The wheel turns at engine rpm divided by the overall ratio, which is the gear ratio times the final drive. Each turn moves the car one tyre circumference, π x diameter. So speed in km/h is wheel rpm x π x diameter (mm) x 60 / 1,000,000. In US units this reduces to the familiar mph = rpm x tyre diameter (in) / (gear x axle ratio x 336), where 336.13 is 63,360 inches per mile divided by 60π.

A metric tyre code gives the section width in millimetres, the aspect ratio as a percentage of that width, and the rim diameter in inches. Overall diameter is rim x 25.4 + 2 x width x aspect / 100 mm, so a 225/45R17 is 431.8 + 202.5 = 634.3 mm. That is the unloaded size. Under load the tyre flattens and rolls on a slightly smaller effective radius, and worn tread is smaller again, which is why makers publish revolutions per mile measured on a rolling tyre.

The rpm drop on an upshift is the shift rpm times the ratio of the next gear over the current one. Close-ratio gearboxes keep that drop small so the engine stays near peak power; wide ratios trade that for a tall top gear that keeps cruising rpm low.

Common use cases

  • Axle ratio and tyre swaps

    See how a different final drive or taller tyres change cruising rpm and the speed in each gear before buying parts.

  • Track and drag gearing

    Check the rpm drop between gears and whether a gear runs out before the end of a straight or the finish line.

  • Engine swaps

    Pair a new engine's redline and power band with the existing gearbox and axle.

  • Speedometer sanity checks

    Compare calculated road speed at a given rpm with what the speedometer shows after a tyre change.

Tips and limitations

  • Unloaded diameter from the size code is slightly larger than the rolling diameter, so real speeds are a little lower than calculated. Use the maker's revolutions per mile figure for precise work.
  • Automatic gearboxes slip through the torque converter until it locks up, so rpm in lower gears is higher than the no-slip figure.
  • Speed at redline in top gear is only a gearing limit. Aerodynamic drag and power decide the real top speed.
  • Check the overall ratio in first gear against the tyre size: very tall tyres with a low-numerical axle can make launching and towing hard.
  • Many speedometers read a few percent high by design, since regulations in many countries forbid them reading below true speed.

Frequently asked questions

How do you calculate speed from RPM and gear ratio?

Speed = engine rpm / (gear ratio x final drive) x tyre circumference. In US units, mph = rpm x tyre diameter (in) / (gear x axle x 336). For 3,000 rpm, a 26 in tyre, 1:1 gear and 3.73 axle that is 62.2 mph.

How do I find tire diameter from the size on the sidewall?

For a size like 225/45R17, diameter = 17 x 25.4 + 2 x 225 x 0.45 = 634.3 mm (24.97 in). The first number is width in mm, the second the sidewall height as a percentage of width, the third the rim size in inches.

What RPM will I be at on the highway?

Divide the speed by the tyre circumference to get wheel rpm, then multiply by the top gear ratio and the final drive. With a 225/45R17, a 0.56 sixth gear and 3.42 final drive, 70 mph is about 1,805 rpm.

Does a higher final drive ratio mean lower top speed?

A numerically higher final drive (for example 4.10 instead of 3.42) means more engine revolutions per wheel turn. It improves acceleration and lowers the speed reached at redline in each gear, and raises cruising rpm.

How do I calculate the RPM drop between gears?

Multiply the shift rpm by the next gear ratio and divide by the current one. Shifting from a 3.36 first gear to a 2.07 second at 6,500 rpm lands at 6,500 x 2.07 / 3.36 = 4,004 rpm.

Why does my speedometer disagree with the calculated speed?

The calculation uses the unloaded tyre diameter, while a loaded tyre rolls on a slightly smaller radius and tread wears down. Speedometers are also calibrated to read a little high. A GPS reading is a better check than the dashboard.

Is there an in-depth guide to the Gear Speed Calculator?

Yes. “Road Speed from RPM, Gear Ratio and Tire Size: The Formula” covers the background, how it works, common pitfalls and worked examples. Read the full guide.

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