How is engine rpm worked out from tire size and gear ratio?

A mile is 63,360 inches, so a tire turns 63,360 divided by its circumference times a mile. Multiply by the speed and divide by 60 for wheel turns a minute, then multiply by the axle ratio and the transmission gear ratio for engine turns a minute. Written out: rpm equals mph times axle ratio times gear ratio times 63,360, divided by 60 times the circumference in inches. For 265/70R17, a 3.73 axle and a 1.00 top gear at 60 mph, that is 2,380 rpm.

What axle ratio gives a bigger tire the same rpm as before?

The current ratio times the new diameter divided by the current diameter. Going from 265/70R17, 31.61 inches tall, to 315/70R17, 34.36 inches, on a 3.73 axle gives 3.73 times 34.36 over 31.61, which is 4.06. The page shows it as the axle ratio for today's engine speed. Axles are made in a fixed set of ratios, so the nearest one offered for your axle is a question for its maker or a gear supplier.

What is the effective gear ratio after a tire change?

The ratio the existing axle behaves like with the new size fitted: the current ratio times the current diameter divided by the new one. A 3.73 axle on 315/70R17 instead of 265/70R17 behaves like a 3.43 on the original tire, which is why a taller tire makes the engine turn slower and pull less hard at any given speed.

How do I find speed from rpm instead?

Use the road speed per 1,000 rpm line. It is the speed the vehicle travels for every 1,000 engine rpm in the gear entered, so 2,500 rpm is 2.5 times that figure. Enter another transmission ratio, such as a first gear of 3.06 or an overdrive of 0.70, to read the same figures in that gear.

Does it work in kilometres per hour?

The speed field is in mph. Engine rpm is proportional to speed, so a km/h figure can be converted first: 100 km/h is 62.14 mph. The tire dimensions themselves can be shown in inches or millimetres with the units switch.

Gear Ratio and Tire Size Calculator

Type the size on the vehicle now and the size you are considering, then the axle ratio, the transmission gear and a road speed. The page works out engine rpm on each size, road speed per 1,000 rpm, the ratio the current axle behaves like once the new size is fitted, and the axle ratio that would turn the engine as fast as it turns today. The tire figures come from the sidewall codes and the rest is the ratios you type, so every number is exact arithmetic with the formula written above the results.

Current225/45R17

Section width: 225 mm (8.86 in)
Sidewall (45%): 3.99 in (101.3 mm)
Rim diameter: 17 in (431.8 mm)
Overall diameter: 24.97 in (634.3 mm)
Circumference: 78.45 in (1992.7 mm)
Revolutions per mile: 808
In inch terms: 25.0x8.9R17

New235/40R18

Section width: 235 mm (9.25 in)
Sidewall (40%): 3.70 in (94.0 mm)
Rim diameter: 18 in (457.2 mm)
Overall diameter: 25.40 in (645.2 mm)
Circumference: 79.80 in (2027.0 mm)
Revolutions per mile: 794
In inch terms: 25.4x9.3R18

Diameter+1.72%

Current size

New size

Units

Reading

Two codes side by side, with the diameter difference and what it does to the speedometer.

Gear ratio and engine speed

Engine rpm = mph x axle ratio x gear ratio x 63,360 / (60 x circumference). Enter the axle ratio and the transmission gear: 1.00 for a direct top gear, below 1 for an overdrive.

Axle ratio

Gear ratio

Speed, mph

Engine speed, current sizeat 60 mph in the gear entered
3012 rpm
Engine speed, new sizeat 60 mph in the gear entered
2962 rpm
Road speed per 1,000 rpmcurrent size, then new size
19.9 / 20.3 mph
Effective axle ratio on the new sizewhat the axle behaves like once the new size is fitted
3.67
Axle ratio for today's engine speedthe ratio that turns the engine as fast as now on the new size
3.79

What the change does

The speedometer and the odometer are out by the same percentage as the diameter, at every speed and in any units: a taller tire turns fewer times per mile, so the needle and the mileage both read low.

Speedometer at 60 mphwhat the needle reads when you are truly doing 60
reads 58.99 mph
Diameter change+1.72% of the current diameter
+0.43 in (10.9 mm)
Axle height changehalf the diameter change: how far the axle centre rises or drops
+0.21 in (5.5 mm)
Odometer over 100 milesmiles it records short (minus) or over (plus) for every 100 actually driven
-1.69 mi

Geometry, not fitment. This tells you what a size measures, not whether it fits your car: it gives no clearance check, no load or speed rating advice and no tire pressure, and it has no vehicle size list. Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.

Common questions

How is engine rpm worked out from tire size and gear ratio?: A mile is 63,360 inches, so a tire turns 63,360 divided by its circumference times a mile. Multiply by the speed and divide by 60 for wheel turns a minute, then multiply by the axle ratio and the transmission gear ratio for engine turns a minute. Written out: rpm equals mph times axle ratio times gear ratio times 63,360, divided by 60 times the circumference in inches. For 265/70R17, a 3.73 axle and a 1.00 top gear at 60 mph, that is 2,380 rpm.
What axle ratio gives a bigger tire the same rpm as before?: The current ratio times the new diameter divided by the current diameter. Going from 265/70R17, 31.61 inches tall, to 315/70R17, 34.36 inches, on a 3.73 axle gives 3.73 times 34.36 over 31.61, which is 4.06. The page shows it as the axle ratio for today's engine speed. Axles are made in a fixed set of ratios, so the nearest one offered for your axle is a question for its maker or a gear supplier.
What is the effective gear ratio after a tire change?: The ratio the existing axle behaves like with the new size fitted: the current ratio times the current diameter divided by the new one. A 3.73 axle on 315/70R17 instead of 265/70R17 behaves like a 3.43 on the original tire, which is why a taller tire makes the engine turn slower and pull less hard at any given speed.
How do I find speed from rpm instead?: Use the road speed per 1,000 rpm line. It is the speed the vehicle travels for every 1,000 engine rpm in the gear entered, so 2,500 rpm is 2.5 times that figure. Enter another transmission ratio, such as a first gear of 3.06 or an overdrive of 0.70, to read the same figures in that gear.
Does it work in kilometres per hour?: The speed field is in mph. Engine rpm is proportional to speed, so a km/h figure can be converted first: 100 km/h is 62.14 mph. The tire dimensions themselves can be shown in inches or millimetres with the units switch.

Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.