Gear Ratio Calculator

How to calculate a gear ratio, step by step

A gear ratio is just one count divided by another: the driven (output) gear's teeth over the drive (input) gear's teeth. Written ratio = driven ÷ drive, a 41-tooth wheel turned by a 13-tooth wheel is 41 ÷ 13 = 3.15, said “3.15 to 1.” The input turns 3.15 times for every single turn of the output, trading speed for torque. That is the whole formula for gear ratio, and it is the same whether you are looking at a gearbox pair, a chain sprocket or an axle. The one thing to get right is which gear is which — flip them and you get the reciprocal — so this drive ratio calculator keeps the boxes clearly labelled driven and drive.

Final drive, axle, differential and ring-and-pinion — one ratio, many names

The final drive ratio is the reduction in the axle, and it goes by a pile of names: axle ratio, differential gear ratio, rear-end gear ratio, or ring-and-pinion ratio. They are all the same number, ring teeth ÷ pinion teeth. A 41-tooth ring on a 13-tooth pinion is 3.15:1; a 37/9 set is about 4.11:1. If the differential tag or your records already give the ratio, type it straight in — you don't have to count teeth. This is the axle ratio calculator and ring-and-pinion gear ratio calculator side of the tool, and the figure it gives is one of the two numbers you need for the overall ratio.

Overall (total) ratio: the number that actually sets your RPM

This is where most people go wrong. The overall ratio is the transmission gear ratio multiplied by the axle ratio: overall = transmission gear × axle. It is the figure that decides engine RPM at a given speed — not the axle on its own. A top gear that is direct is 1.00:1; an overdrive top gear is below 1.00, often 0.70 to 0.85. So a 3.73 axle with a 0.73 overdrive gives an overall of only 2.72, and if you drop the axle alone into the speed formula your RPM comes out wildly high. Always combine the gear you are actually in with the axle. The transmission-set table in the first mode does this for every gear at once.

Crawl ratio for low-range four-wheel drive

Off-road, the number that matters at walking pace is the crawl ratio — the total reduction with everything stacked in low range: crawl = transmission low × transfer-case low × axle. A 4.71 first gear, a 4.0:1 low-range case and a 4.10 axle multiply to about 77:1. The higher it is, the slower and more controllable the vehicle creeps over obstacles at idle, with less brake and clutch work. Builds commonly aim somewhere between 50:1 and 90:1 depending on tire size and use. Enter your own three figures and the tool multiplies them.

Speed ↔ RPM in any gear

The standard relationship between road speed, engine RPM, gearing and tire size is:

MPH = (RPM × tire diameter in inches) ÷ (336 × overall ratio)
RPM = (MPH × overall ratio × 336) ÷ tire diameter

The 336 is a unit constant: 63,360 inches in a mile, divided by 60 minutes, divided by π. The ratio in these formulas is the overall ratio (transmission gear × axle), which is exactly the trap that produces wrong answers when only the axle is used. The Speed ↔ RPM mode rearranges the formula for whichever box you leave blank, so it works as a gear ratio speed calculator, a gear ratio RPM calculator, or even a top-speed gear ratio calculator if you put in your redline. Typical highway cruise sits around 1,800–2,500 RPM in top gear.

What bigger tires do to gearing and your speedometer

A taller tire rolls further per revolution, so it behaves like a numerically lower axle — it lowers your effective gearing, which is why a truck feels slower and turns fewer RPM after a tire upsize. The effective ratio is:

effective ratio = (old diameter ÷ new diameter) × original ratio
actual speed = indicated speed × (new diameter ÷ old diameter)
% speedometer error = (new − old) ÷ old × 100

Because the wheel turns slower for the same road speed, the speedometer reads low after fitting bigger tires (and reads high after fitting smaller ones), and the odometer, cruise control and automatic shift points drift with it. This is the tire size gear ratio calculator and speedometer error side of the tool: enter old and new diameters — directly or computed from a tire size — and it gives the effective gearing and the true speed at any indicated reading.

Regearing to restore stock gearing

To put the engine back where it was for a given speed, you regear by the same proportion the tire grew:

new ratio = stock ratio × (new diameter ÷ old diameter)

With a 3.73 axle and a jump from a 28-inch to a 33-inch tire, 3.73 × 33 ÷ 28 ≈ 4.40, so a 4.40 — or the nearest you can buy, often 4.56 — restores roughly stock performance and cruise RPM. The tool gives the exact figure and you round to an available ratio.

Tire diameter from a tire size

If you only know the sidewall size rather than a measured diameter, the calculator computes it:

diameter (in) = wheel (in) + 2 × (section width mm × aspect ÷ 100) ÷ 25.4

For a 285/75R16, that's 16 + 2 × (285 × 0.75) ÷ 25.4 ≈ 32.8 inches. This is a theoretical, unloaded figure — real mounted height varies with wheel width, pressure, load and brand, so a measured diameter is always better when you have one.

A worked example

You're cruising at 70 MPH in a 0.73 overdrive top gear behind a 3.73 axle, on 28-inch tires.

• Overall ratio = 0.73 × 3.73 = 2.72 (using the axle alone, 3.73, would overstate RPM by a third).
• RPM = 70 × 2.72 × 336 ÷ 28 ≈ 2,286 RPM — a comfortable highway cruise.
• Fit 33-inch tires and the effective ratio drops to 28 ÷ 33 × 2.72 ≈ 2.31, RPM falls and the speedometer reads about 18% slow.
• To restore it, regear to 3.73 × 33 ÷ 28 ≈ 4.40 (round to 4.56).

And a crawl example: a 4.71 first gear, a 4.0 low-range case and a 4.10 axle give 4.71 × 4.0 × 4.10 ≈ 77:1.

Terms, in plain English

A note on spelling and units for readers outside the US: “tire” and “tyre” are the same thing, “gearing” and “gear ratio” are used interchangeably, and the tool will work in inches and MPH or in millimetres and km h⁻¹. The arithmetic is identical; only the labels differ.