Calculator 02 / Gear Ratio & Tires
Gear Ratio & Tire Calculator
Engine speed at a given road speed depends on axle ratio, transmission ratio and tyre diameter together — changing any one of the three moves the others' effect. Find engine RPM at a given road speed for your gear ratio and tire size, or how much a tire size change throws off your speedometer.
How this calculator works
Units below default automatically — Metric for non-English pages (or if your device's language is set to one we support), US customary otherwise — and convert automatically if you switch the toggle above the form. Changing language resets to that language's default unless you toggle again.
The 336 constant in the RPM formula comes from converting tire revolutions per mile into RPM at a given road speed, assuming no tire slippage. It's the same constant used across most off-road and gearing calculators. For speedometer error: most factory speedometers are calibrated to the stock tire's rolling diameter, so fitting larger tires makes your actual speed higher than what's displayed (and smaller tires make it read fast) — useful to know before trusting cruise control or a speed-dependent inspection.
Worked example: 65 mph on a 3.73 axle
65 mph, a 32.1 in tire, a 3.73 axle ratio and a 1.00 transmission gear — direct drive:
- Total gear ratio = 3.73 × 1.00 = 3.73:1
- RPM = (65 × 3.73 × 336) ÷ 32.1
- = 81,463 ÷ 32.1 = 2,538 RPM
The 336 is not arbitrary. A tire turns 63,360 ÷ (π × diameter) times per mile, and there are 60 minutes in an hour, so 63,360 ÷ 60 ÷ π = 336.1. Everything else in the formula is your own drivetrain.
Cruise RPM by axle ratio
All at 65 mph on a 32.1 in tire in direct drive, so the only thing changing is the axle. This is the trade every spec decision comes down to: a numerically lower ratio cruises slower and burns less fuel, a higher one pulls better and accelerates harder.
| Axle ratio | RPM at 65 mph | Character |
|---|---|---|
| 2.64 | 1,796 | Very tall — fuel-economy highway spec |
| 2.79 | 1,898 | Tall highway spec |
| 3.08 | 2,096 | Common on-highway compromise |
| 3.36 | 2,286 | Mixed on- and off-highway |
| 3.55 | 2,415 | General purpose |
| 3.73 | 2,538 | Towing and heavier work |
| 4.10 | 2,790 | Heavy haul, steep terrain |
| 4.30 | 2,926 | Vocational, low-speed work |
Between 2.64 and 4.30 there is a 1,130 RPM spread at the same road speed. Modern on-highway diesels are designed to cruise low, and an engine spinning several hundred RPM above its sweet spot all day is the difference between good and poor fuel economy on the same route with the same driver.
Turning a tire size into a diameter
The calculator wants a diameter in inches, and tires are almost never labelled that way. For a metric size, diameter = 2 × (section width × aspect ratio ÷ 100) ÷ 25.4 + rim diameter. These are the nominal figures from that arithmetic.
| Size | Nominal diameter | Typically on |
|---|---|---|
| 245/75R16 | 30.5 in | Light truck, older 3/4-ton |
| LT265/70R17 | 31.6 in | Modern 3/4- and 1-ton pickup |
| 235/85R16 | 31.7 in | Utility and older heavy-duty pickup |
| 285/70R17 | 32.7 in | Common upsize on a 17 in wheel |
| 275/80R22.5 | 39.8 in | Class 8 drive and trailer |
| 295/75R22.5 | 39.9 in | Class 8 steer and drive |
Nominal is not loaded. A tire carrying weight squats, so its rolling diameter is smaller than the arithmetic above — commonly by 2–3%. It also shrinks as the tire wears and changes with inflation pressure. Where the answer has to be right, measure the loaded radius or use the manufacturer's revolutions-per-mile figure, which already accounts for all of it.
What a tire change does to the speedometer
A speedometer counts driveshaft or wheel revolutions and multiplies by an assumed tire size. Fit a bigger tire and every revolution covers more ground, so you are going faster than the needle says — and the odometer is undercounting by the same percentage.
| Stock | Fitted | True speed at an indicated 65 | Error |
|---|---|---|---|
| 28.9 in | 30.5 in | 68.6 mph | +5.5% |
| 28.9 in | 31.6 in | 71.1 mph | +9.3% |
| 28.9 in | 32.1 in | 72.2 mph | +11.1% |
| 28.9 in | 33.0 in | 74.2 mph | +14.2% |
| 28.9 in | 35.0 in | 78.7 mph | +21.1% |
The last row is a 35 in tire on a truck that left the factory on 28.9 in: an indicated 65 is really 78.7 mph. That is a speeding ticket the driver does not know they are earning, and over a year the odometer under-records by more than a fifth — which flows straight into service intervals, warranty terms, fuel-tax reporting and what you disclose at resale. Fitting larger tires does not recalibrate anything on its own.
Where this goes wrong
- Using the tire's nominal diameter as if it were loaded. It is 2–3% optimistic before you account for wear.
- Leaving the transmission ratio at 1.00 when you are not in direct drive. Overdrive is below 1.00 and drops the RPM; every lower gear is above 1.00 and raises it. The field exists precisely so the answer is not always a top-gear answer.
- Forgetting the auxiliary or splitter box. If there is another reduction in the driveline, multiply it in — the calculator only has two ratio fields.
- Assuming an automatic is locked up. A torque converter that is not locked slips, so engine RPM runs above what the gearing implies.
- Treating speedometer error as harmless. An inaccurate speedometer is a vehicle defect in many jurisdictions, and the odometer error compounds every mile into records other people rely on.
- Comparing a stock and a fitted tire in different units. The speedometer tab uses the ratio of the two diameters, so it does not care whether you work in inches or millimetres — but it does care that both boxes use the same one.
Where these numbers come from
A tire size change moves engine RPM, indicated speed and effective gearing together. The references below are for the standards the numbers on a sidewall come from.
- U.S. Tire Manufacturers Association — How the size designation on a sidewall maps to a real rolling diameter, and why substitutions change more than looks.
- SAE International — The engineering standards behind drivetrain, tire and vehicle-performance measurement.
Why does tire diameter matter more than just the tire's labeled size?
Two tires with different tread patterns or aspect ratios can have the same nominal size label but different actual overall diameters — measure or look up the actual diameter for an accurate result.
What transmission ratio should I use for a general "cruising RPM" check?
Use 1.00 if your top gear is a direct 1:1 (common on many manual and automatic transmissions), or check your transmission's spec sheet if it has an overdrive ratio below 1.00.
Estimates only, and assume zero tire slippage. Confirm your vehicle's actual axle ratio and tire diameter before making gearing decisions.