Gear Ratio Calculator
Compute a gear ratio from drive and driven tooth counts, multiply by final drive for overall ratio, or compare two gears to see the percentage drop — handy for transmission and differential work.
Calculate
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Result
The formula
A simple gear pair ratio is driven teeth ÷ drive teeth (how many turns the input needs for one output turn when greater than 1). Overall ratio multiplies transmission gear by final drive. Percentage drop between gears is (ratio₁ − ratio₂) ÷ ratio₁ × 100%.
Worked example
- Ring 41 teeth, pinion 11 teeth → i = 41 ÷ 11 ≈ 3.727
- 3rd gear 1.45 × final 3.727 → overall ≈ 5.40
- 2nd 2.10 vs 3rd 1.45 → drop = (2.10 − 1.45) ÷ 2.10 ≈ 31%
Result: Axle ≈ 3.73:1 · overall in 3rd ≈ 5.40:1 · 2→3 drop ≈ 31%
How gear ratios are calculated
Tooth-count math assumes meshing spur/helical pairs without intermediate idlers changing the ratio magnitude. Planetary sets need a different model.
Drive vs driven
“Drive” is the input gear (pinion); “driven” is the output (ring). For a differential, ring ÷ pinion is the axle ratio stamped on many housings (e.g. 3.73).
Overall ratio
Wheel torque multiplies by transmission ratio × final drive (and any transfer case). Higher numeric overall ratio means more torque multiplication and higher engine RPM at a given road speed.
Percentage drop
Drop between consecutive gears describes how far RPM falls on an upshift at constant speed. Large drops can bog a peaky engine; close ratios keep the engine in its power band.
What we do not model
No planetary gearsets, CVTs, dual-clutch slip, or efficiency losses — ratio geometry only.
Interesting facts
Why 3.73 and 4.10
Popular axle ratios trade cruise RPM against launch. A 4.10 turns the engine faster than a 3.73 at the same highway speed with the same tire.
Close-ratio boxes
Sport and race gearboxes use smaller percentage drops so upshifts stay near peak power — at the cost of more shifts for a given speed range.
Overdrive means < 1
When transmission ratio is below 1.0, output spins faster than input — lower cruise RPM, less torque multiplication.
Count the teeth
If paperwork is missing, counting ring and pinion teeth still yields the axle ratio — a common shop method.
Frequently asked questions
Divide driven (output) tooth count by drive (input) tooth count. Example: 41 ÷ 11 ≈ 3.73:1.
Transmission gear ratio multiplied by final drive (and transfer case if present). It links engine turns to wheel turns.
Street boxes often see ~20–40% drops. Exact targets depend on engine torque curve and use — track cars usually prefer tighter spacing.
Higher numeric ratios multiply torque and raise RPM at speed — better for acceleration or towing, worse for relaxed highway revs.
Yes for a single chainring/cog pair (teeth ÷ teeth). Derailleur gear-inches need wheel size too — use RPM/speed tools for road speed.
References
- Fundamentals of vehicle dynamics Driveline ratio definitions used in automotive engineering.
- Gear nomenclature Industry terminology for gear geometry and ratios.