How to Calculate Gear Ratio: The RPM Formula, Tables and What Changes
If you want a straight answer on how to calculate gear ratio, start with the formula: RPM equals speed times gear ratio times final drive times 336, divided by tyre diameter in inches. That one formula answers every gear-ratio question on this page — what RPM you will see at a given speed, what speed a given RPM means, and what ratio you need when the tyre size changes. This page works the formula step by step, gives RPM tables at 30, 50 and 70 mph, and explains what the numbers mean for acceleration, towing and cruising.
This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.
how to calculate gear ratio: The RPM formula
Taking how to calculate gear ratio as the case in point: RPM = (speed × gear ratio × final drive × 336) ÷ tyre diameter (inches) is the whole method in one line. Speed is in mph, the gear ratio is the transmission gear in use — 1.00 in direct drive, under 1.00 in overdrive, 0.69 for a typical overdrive gear — the final drive is the axle ratio, and 336 is the constant that makes the units work. Turn it round for speed: speed = (RPM × tyre diameter) ÷ (gear ratio × final drive × 336). Diameters here are nominal (new-tyre) figures, and real tyres vary slightly by brand, model, wear and inflation pressure, so take the diameter from the tyre-size chart for your size: a 265/70R17 measures 802.8 mm (31.61 in) across.
Worked example: RPM at 30, 50 and 70 mph for how to calculate gear ratio
On this page, how to calculate gear ratio is the case in point: Take a 265/70R17 tyre with a nominal diameter of 802.8 mm (31.61 in) and a 3.73 axle ratio, in direct drive (gear ratio 1.00). At 70 mph, RPM = (70 × 1.00 × 3.73 × 336) ÷ 31.61 = 2,775. At 50 mph the same sum gives 1,982 RPM, and at 30 mph it gives 1,189 RPM. In overdrive — a 0.69 gear ratio, as an example — the 70 mph figure falls to 1,915 RPM, which is why motorway cruising feels relaxed in top gear. Change any one input and the answer moves in proportion: a bigger tyre lowers RPM, a higher ratio raises it, and a taller overdrive gear lowers it again.
how to calculate gear ratio: RPM table: 30, 50 and 70 mph
On this page, how to calculate gear ratio is the case in point: RPM for a 3.73 axle ratio on a 265/70R17 tyre (31.61 in diameter). The direct-drive column uses a 1.00 gear ratio; the overdrive column uses 0.69 as an example.
| Speed (mph) | RPM in direct drive (1.00) | RPM in overdrive (0.69) |
|---|---|---|
| 30 | 1,189 | 821 |
| 50 | 1,982 | 1,368 |
| 70 | 2,775 | 1,915 |
Speed at fixed RPM for how to calculate gear ratio
Taking how to calculate gear ratio as the case in point: Rearranged, the formula gives speed from RPM. With the same 3.73 ratio and 31.61-inch tyre, 2,000 RPM in direct drive means 50.4 mph, and 3,000 RPM means 75.7 mph. In overdrive the same 2,000 RPM means 73.1 mph. That is the quickest way to sanity-check a ratio you have found on an axle tag: drive at a steady speed in a known gear, note the RPM, and see whether the formula's answer matches the speedometer. If it does not, either the ratio is not what you thought, the tyre diameter is not what you assumed, or the speedometer itself is off — all three are worth checking before spending money on gears.
What the gear ratio changes for how to calculate gear ratio
A numerically higher ratio — 4.10 rather than 3.21, for example — multiplies the engine's torque more before it reaches the wheels, so the vehicle accelerates harder and pulls a trailer more easily, but the engine turns faster at any given road speed. A numerically lower ratio does the opposite: relaxed cruising and lower RPM on the motorway, but less pull off the line and more downshifting on hills or when towing. Fuel use follows the same split in general terms: higher cruising RPM usually means more fuel on the motorway, while a ratio that is too low for the tyre size can also waste fuel, because the engine labours below its efficient range and the transmission downshifts constantly. Bigger tyres have the same effect as a lower ratio — they lower the RPM at a given speed — which is why a ratio change is the standard correction after fitting larger tyres. The right ratio is the one that puts the engine in its useful rev range for the way the vehicle is actually driven, not the one with the biggest number. That is the point this page works through for how to calculate gear ratio.
how to calculate gear ratio: Common mistakes
- For how to calculate gear ratio, the point is this: Using the rim size as the tyre diameter. A 17-inch rim is not a 17-inch tyre: the overall diameter includes both sidewalls, so a 265/70R17 is about 31.6 inches across, and using 17 inches in the formula gives nonsense.
- On this page, how to calculate gear ratio is the case in point: Changing ratios without recalibrating the speedometer. The speedometer, odometer and shift points all depend on the ratio and tyre size, so a gear change without recalibration leaves every reading wrong.
- Mixing up high and low ratios. A numerically higher ratio (4.10) is a 'lower' gear in driving terms — more torque, higher RPM — while a numerically lower ratio (3.21) is a 'higher' gear. The wording trips people up every time. That is the point this page works through for how to calculate gear ratio.
- On this page, how to calculate gear ratio is the case in point: Forgetting that front and rear ratios must match on a four-wheel-drive vehicle. Different ratios front and rear will fight each other in four-wheel drive and damage the transfer case, so both axles have to be changed together.
how to calculate gear ratio: Related calculations
For the next step, these pages apply the same method to a closely related question — or hand the arithmetic to the calculator. That is the point this page works through for how to calculate gear ratio.
- 3.21 Gear Ratio
- Gear Ratio Calculator 4lo
- Gear Ratio Calculator 12 Bolt
- What Is 3.73 Gear Ratio
- Use the calculator
- Browse all gear ratio pages
how to calculate gear ratio: Putting your figure to work
Taking how to calculate gear ratio as the case in point: The formula is the cheap part of the job, so use it first. Work out your RPM at real driving speeds with your tyre diameter from the chart and your axle ratio from the tag, and compare it with where the engine feels comfortable. If a change is needed, get a full quote — gears, labour and recalibration — and have a drivetrain specialist confirm the ratio before ordering. A ratio picked from a table without the RPM check is a guess, and guesses in gearing are expensive to undo.
Frequently asked questions
What is the difference between gear ratio and final drive ratio for how to calculate gear ratio?
They are the same idea at different places. The gear ratio is the transmission gear you are in; the final drive ratio is the axle ratio. The formula multiplies them together, so the total reduction from engine to wheels is the product of the two. That is the point this page works through for how to calculate gear ratio.
What happens if my gear ratio is too low?
Taking how to calculate gear ratio as the case in point: The engine feels gutless, the transmission hunts between gears on hills, and acceleration from a standstill is sluggish — especially with bigger tyres. The fix is usually a numerically higher ratio, fitted by a drivetrain specialist with the speedometer recalibrated afterwards.
How accurate is the RPM formula?
For how to calculate gear ratio, the point is this: It is exact for the inputs you give it: speed, gear ratio, final drive and tyre diameter. The uncertainty comes from the inputs, mainly the tyre diameter, which is a nominal new-tyre figure that varies slightly with brand, wear and pressure. A few percent either way is normal.
Can I change just the rear axle ratio on a 4x4?
For how to calculate gear ratio, the point is this: No — on a four-wheel-drive vehicle the front and rear ratios must match, or the axles will fight each other in four-wheel drive and damage the transfer case. Both axles have to be changed together, which is why a 4x4 gear change costs more than a two-wheel-drive one.
How do I find my gear ratio?
For how to calculate gear ratio, the point is this: Check the axle tag on the differential cover, the door-jamb sticker, the build sheet, or ask a dealer to decode the VIN. You can also count driveshaft turns against wheel turns, or check the ratio against the RPM formula at a steady speed.