Gear Ratio Calculator
Put in your tyre size, gearbox ratios and final drive, and see the road speed at any engine rpm, gear by gear. Change the axle ratio and watch what it does to cruising revs.
Calculated from the numbers you enter, using rolling diameter from the tyre size, the gearbox ratio and the final drive. Real road speed differs a little: tyres deflect under load and grow slightly at speed, and speedometers read optimistically by design. Torque converters add slip in the automatic gears until lockup.
What this calculator actually computes
There is no vehicle database behind this page and nothing is looked up. Every figure comes from the four numbers you enter, through arithmetic you can check by hand:
circumference = π × diameter
revs per mile = 63,360 ÷ circumference
mph = (rpm × circumference × 60) ÷ (gear × final drive × 63,360)
With the defaults loaded — 225/45R17, a 3.73 axle and a 0.65 sixth gear — the tyre works out to 24.97 in across, 808 revolutions per mile, and 61 mph at 2,000 rpm. The 63,360 is just inches in a mile, and the 60 converts revolutions per minute to revolutions per hour.
Because it is pure arithmetic, it works for a car that does not exist yet. That is the point: you can price a differential swap or a tyre size before buying either.
Finding your final drive ratio
The ratio is rarely printed anywhere obvious, and the wrong one makes every number on this page wrong. Three ways to get it, in order of how much effort they cost:
- The build sticker. Ford puts an axle code on the door jamb label; GM lists an RPO axle code on the service parts identification label, usually in the glovebox. Both need a decoder table from the owner’s manual or the service literature — the code itself is not the ratio.
- The tag on the differential. Many live axles carry a metal tag under one of the cover bolts with the ratio stamped on it, often as a tooth count such as 41–11, which is 3.73.
- Count it. Chock the front wheels, raise both rear wheels, put the transmission in neutral, and mark the driveshaft and one tyre. Hold one wheel still and turn the other through exactly two full revolutions. The number of driveshaft turns is the ratio. Two turns rather than one, because an open differential sends half the movement to the wheel you are holding.
The presets are typical, not yours
The gearbox sets in the dropdown are ordinary ratios for their type, and they are there so the page is useful before you have found your own. They are not a lookup for your car.
Real ratios live in the service manual, usually in the transmission specification table. Type them into the boxes and the preset switches itself to Enter my own. Clearing a box drops that gear, and the eight-speed preset is there when you need more boxes to fill, so a four-speed works as well as an eight-speed.
One thing worth knowing before you compare: top gear ratios below 1.00 are overdrives, and almost every modern gearbox has at least one. A 0.65 sixth means the driveshaft turns faster than the engine.
Where the calculated speed and the real one part company
The arithmetic assumes a rigid tyre rolling without slip. A real one does neither, so expect the road speed to land a few percent below the calculated figure:
- The tyre squashes. Loaded rolling radius is smaller than half the moulded diameter, which is why tyre makers publish a measured revs-per-mile figure rather than the calculated one. Low pressure makes the gap wider, so it is worth knowing your placard pressures.
- And then it grows. Centrifugal force stretches the tread at speed, partly cancelling the deflection. The two effects do not cancel neatly at any particular speed.
- Automatics slip. Until the torque converter clutch locks, engine rpm runs above what the gear ratio alone would give. Above lockup the arithmetic holds.
- Speedometers are optimistic on purpose. Regulations in most markets allow a speedometer to read high but never low, and manufacturers use that allowance.
None of this makes the calculation useless. It makes it a comparison tool: the difference between a 3.73 and a 4.10 is reliable even when the absolute mph is a couple of percent out.
Frequently asked questions
Does a numerically higher axle ratio make the car faster?
It makes it quicker off the line and slower on the motorway. Going from 3.73 to 4.10 multiplies torque at the wheels by about ten percent, and raises engine speed at any given road speed by the same ten percent — 2,284 rpm becomes 2,511 rpm at 70 mph on the default setup. You reach each gear’s rev limit at a lower speed, so top speed usually drops and fuel consumption at a cruise rises.
I fitted taller tyres and my speedometer is wrong. By how much?
Actual speed = indicated speed × new diameter ÷ old diameter. Going from 24.97 in to 26.0 in means an indicated 70 mph is really about 72.9. Most cars derive speed by counting pulses from the wheel speed sensors and multiplying by a tyre circumference stored in software, so the fix is recalibration, not a new sensor. The odometer and any speed-based shift or stability logic drift by the same percentage.
What is the difference between gear ratio and final drive?
The gear ratio is inside the transmission and changes every time you shift. The final drive is the fixed reduction in the differential, between driveshaft and wheels. Multiply the two and you get the total drive ratio, which is the number that actually sets road speed: 0.65 × 3.73 = 2.42:1.
Why will my car not reach the speed shown for top gear at redline?
This page answers what road speed corresponds to a given engine speed, not whether the engine can get there. Reaching it needs enough power to overcome aerodynamic drag and rolling resistance at that speed, and drag rises with the square of velocity. Many cars are geared for a theoretical top speed they have no chance of achieving, which is normal and deliberate — it keeps cruising revs down.
Can I use this for a motorcycle, a tractor or a go-kart?
Yes, as long as you can express the tyre in the metric width/aspect/rim format and you know both ratios. For chain drive, enter the sprocket reduction — rear teeth divided by front teeth — in the final drive box. The arithmetic does not care what the vehicle is.
Does the calculator account for the transfer case on a four-wheel drive?
Not separately. High range is normally 1.00:1 and can be ignored. For low range, multiply your final drive figure by the transfer case reduction and enter the result — a 3.73 axle with a 2.72:1 low range behaves as 10.15:1.