RPM to MPH Calculator - Engine Speed to Road Speed

Rpm to mph, and mph back to rpm, for one gear

Is 2,000 rpm at 70 mph a lot or a little? The tachometer cannot say on its own, because the same engine speed moves a car at very different road speeds depending on the gear, the axle and the tire. This rpm to mph calculator ties the three together. Give it the engine rpm and it returns the speed in mph and km/h; give it a speed and it returns the rpm. Two tables show the whole picture for that gear: road speed from 1,000 to 7,000 rpm, and the rpm at 25 to 80 mph.

Problem: "rpm at 70 mph" means nothing without three other numbers

One truck turns 1,610 rpm at 70 mph, another 2,139, and both can be exactly as built. The first has a 2.26:1 overall ratio in top gear on 33-inch tires, the second 3.00:1. Comparing rpm figures between vehicles, or before and after a tire change, only works when the gear, the axle and the tire diameter come along with them.

Solution: put the gearing and the tire on the same page

What you know - engine rpm (from the tachometer or a log) or road speed, in mph or km/h.
Gear ratio - the gear you are in, from 0.30 to 7.00. A 1.00 is direct drive; anything below it, like 0.636, is an overdrive.
Axle ratio - the final drive, from 1.50 to 7.00, such as 3.55 or 4.10.
Tire size - metric (275/60R20) or flotation (35x12.50R17, where 35 is the diameter in inches).

The calculator multiplies the gear by the axle, turns the tire size into a diameter, and applies mph = rpm x diameter / (gear x axle x 336.14). Going the other way, rpm = mph x gear x axle x 336.14 / diameter. Ratios go in as printed in the spec sheet, such as 0.636.

Rpm at 70 mph for five overall ratios and five tire sizes

Overall ratio is the top gear times the axle. Read across for a bigger tire on the same gearing, down for a numerically higher axle on the same tire. The 2.25 row is the one closest to the 0.636 x 3.55 = 2.26 example used below, which gives 1,610 on a 33.0-inch tire.

Overall ratio	26 in	29 in	31 in	33 in	35 in
2.00	1,810	1,623	1,518	1,426	1,345
2.25	2,036	1,826	1,708	1,604	1,513
2.50	2,262	2,028	1,898	1,783	1,681
3.00	2,715	2,434	2,277	2,139	2,017
3.50	3,167	2,840	2,657	2,496	2,353

Six setups run through the calculator

Ten-speed truck, 10th gear. 0.636 x 3.55, 275/60R20 (33.0 in), 70 mph: 1,610 rpm, or 23 rpm for every mph.

Same truck, one gear down. 9th at 0.689, same axle and tire, 70 mph: 1,744 rpm.

Same gearing on 35-inch tires. At 2,000 rpm in 10th the 33.0-inch tire gives 86.9 mph and the 35-inch one 92.2 mph: the bigger tire moves the truck farther per revolution.

Six-speed manual in 6th. 0.63 x 3.73, 245/45R18 (26.7 in), 70 mph: 2,072 rpm. Small tires and a numerically higher axle add up.

Direct gear off road. 1.00 x 4.10 on 35-inch tires at 2,500 rpm: 63.5 mph.

A metric speed limit. The first truck at 120 km/h (74.6 mph) in 10th: 1,715 rpm.

Rpm per mph in direct drive, by tire diameter

For a quick estimate without the calculator: rpm per mph = overall ratio x 336.14 / diameter. The table gives that number for an overall ratio of 1.00, so multiply it by your gear x axle, then by the speed.

Tire diameter	Revs per mile	Rpm per mph at 1.00:1
26 in	776	12.93
29 in	695	11.59
31 in	651	10.84
33 in	611	10.19
35 in	576	9.60
37 in	545	9.08

Example: a 33-inch tire and 0.636 x 3.55 = 2.26. Then 10.19 x 2.26 = 23 rpm per mph, and 23 x 70 = about 1,610 rpm.

What drivers ask about rpm and speed

Why is my real rpm higher than the calculator shows?

Usually slip. An automatic with the torque converter unlocked lets the engine run faster than the gearbox input, and a slipping clutch does the same in a manual. A loaded tire also rolls slightly smaller than its nominal size, which adds a little rpm at a given speed.

How do I find which gear ratio I am in?

The owner's manual or the transmission spec sheet lists every forward ratio. If you only know the gear number, look up that transmission's ratios; the Transmission Gear Ratio Calculator then shows every gear at once.

Can I work out my axle ratio from rpm and speed?

Yes, by rearranging the formula: axle = rpm x diameter / (mph x gear x 336.14). Read rpm and a GPS speed together in a gear with a locked converter or a fully engaged clutch, for example 1,610 rpm at 70 mph in a 0.636 gear on 33-inch tires points to about 3.55.

Does rpm to mph work for a motorcycle or a bicycle?

For a motorcycle, yes, if you enter the primary and final reductions multiplied together as the axle ratio. For pedaling cadence on a bike, the Cycling Cadence Calculator is the right tool, since it works from chainring and cog teeth.

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