Gear ratios decide how an engine’s speed is traded for torque at the wheels. The ring and pinion in the axle set the final drive ratio, the transmission adds its own ratio in each gear, and together they determine both how hard the car pulls and how fast the engine spins on the motorway. This calculator computes the axle ratio from the tooth counts, the overall ratio with a chosen transmission gear, and the road speed for a given engine speed and tire size.
How to use the gear ratio calculator
- Enter the ring and pinion tooth counts (for example 41 and 11). If you only know the ratio, such as 3.73, enter 373 and 100.
- Enter the transmission ratio for the gear you care about. Use 1.00 for a direct top gear, a number below 1 for an overdrive.
- Enter the overall tire diameter — calculate it from the sidewall with the tire diameter calculator.
- Enter the engine speed to see the road speed at that rpm.
To go the other way, finding rpm at a given speed, use the engine RPM calculator.
Gear ratio formulas
Overall ratio = final drive ratio × transmission gear ratio
Speed (in/min) = rpm × π × tire diameter ÷ overall ratio
Speed (mph) = speed (in/min) × 60 ÷ 63,360
A ratio of 3.73:1 means the driveshaft turns 3.73 times for every single turn of the wheels. The wheel turns once per circumference of travel, so the road speed is the engine speed divided by the overall ratio, times the tire circumference. Power losses in the drivetrain and tire squash are ignored.
Worked example: 41:11 axle, direct top gear
A truck has a 41-tooth ring gear and an 11-tooth pinion, a top gear ratio of 1.00 and 28-inch tires. The engine turns at 2,000 rpm.
- Final drive ratio: 41 ÷ 11 = 3.727:1.
- Overall ratio: 3.727 × 1.00 = 3.727:1.
- Tire circumference: π × 28 = 87.96 in.
- Wheel speed in inches per minute: 2,000 × 87.96 ÷ 3.727 = 47,200.
- Speed: 47,200 × 60 ÷ 63,360 = 44.7 mph.
That is a low figure because top gear is direct (1.00); a modern overdrive of 0.70 would raise it to 63.9 mph at the same rpm.
Common axle ratios compared
Speed at 2,000 rpm, in a 1:1 top gear on 28-inch tires. Each row is calculated.
| Ring : pinion teeth | Final drive ratio | Speed at 2,000 rpm in top gear (28 in tire, 1:1) |
|---|---|---|
| 37:11 | 3.36 | 49.5 mph |
| 41:11 | 3.73 | 44.7 mph |
| 43:11 | 3.91 | 42.6 mph |
| 41:10 | 4.10 | 40.6 mph |
| 43:9 | 4.78 | 34.9 mph |
Choosing a ratio
A numerically high ratio (4.10, 4.56) multiplies engine torque more and is common on towing, off-road and drag setups. A numerically low ratio (2.73, 3.08) lets the engine loaf along on the highway and improves fuel economy, but gives lazier acceleration and weaker towing. Because the choice interacts with tire diameter, changing from 28-inch to 33-inch tires is roughly equivalent to moving to a ratio 15% lower numerically — a reason many truck owners re-gear after a tire upgrade.
Keep the pinion tooth count practical: very small pinions in a 3.73 set mean larger pinion teeth and more strength, while “fine” sets use more teeth in a given gear size and run quieter. Ring and pinion sets are lapped as a matched pair, so always replace them together, and set the pinion depth and backlash to the manufacturer’s specification.
Sources
Gear ratio is defined as the ratio of the driven gear’s tooth count to the driver’s; the road-speed formula uses 63,360 inches per mile (exact). Check axle codes and transmission ratios in the vehicle’s service manual.
Frequently asked questions
How do I calculate a gear ratio?
Divide the number of teeth on the driven gear (the ring gear) by the number of teeth on the driving gear (the pinion). A 41-tooth ring and an 11-tooth pinion give 41 ÷ 11 = 3.73:1.
Is a higher gear ratio better?
A numerically higher ratio (such as 4.10) gives stronger acceleration and towing ability but a lower top speed per rpm and higher engine speed at cruise. A lower ratio (such as 3.08) favours highway economy.
How do I find my axle ratio?
Look for the axle code on the vehicle’s build sheet or door-jamb label, count the turns of the driveshaft per one turn of a wheel with the rear wheels lifted, or read the tooth counts stamped on the ring and pinion.
What is the overall gear ratio?
The transmission ratio multiplied by the axle ratio. In a 1.00 top gear the overall ratio equals the axle ratio; in first gear (for example 3.06) it is far higher.
Why does a bigger tire change the effect of the gearing?
A taller tire covers more ground per revolution, which acts like a numerically lower axle ratio. If you fit larger tires, you can restore the original effect with a lower (numerically higher) axle ratio.