Gear Ratio Calculator
Calculate gear ratio from tooth counts and RPM at any speed. Useful for car differential setup, bicycle gearing, and machinery design.
Reviewed & updated for 2026 by Rakesh Choudhary, PhD · How we calculate
The torque-speed tradeoff every driver should understand
A gear ratio is a torque multiplier and a speed divider, and that tradeoff is non-negotiable. A 4:1 ratio quadruples the input torque at the output but cuts output rotational speed to one-quarter of the input. There is no free lunch in mechanical advantage; energy in equals energy out (minus friction losses, typically 5-15% per gear pair).
In a car, your engine makes peak power in a narrow RPM band (often 5,000-6,500 RPM for gasoline engines, 3,000-4,500 for diesels). The transmission and final drive's job is to keep the engine in that band across all road speeds. First gear's job is to multiply torque enough to overcome a stopped vehicle's inertia. Top gear's job is to cruise efficiently at highway speed without screaming the engine to redline.
Total ratio = transmission ratio × final drive ratio. A car with a 1.0 top gear and 3.55 differential has a 3.55 total ratio. A truck with a 4:1 low gear and 4.10 differential has a 16.4:1 ratio in first, meaning the engine spins 16.4 times for every wheel rotation. That's why pickups can pull stumps and sports cars can hit 200 mph: same engine, very different gearing.
Worked example: regearing for 35-inch tires
Suppose you have a Jeep Wrangler with stock 31-inch tires and 3.73 differential gears, cruising comfortably at 2,200 RPM at 65 mph. You install 35-inch tires for off-road clearance, bigger tires mean each revolution travels further, dropping effective gearing.
New effective ratio = old ratio × (old tire / new tire) = 3.73 × (31/35) = 3.30. At 65 mph the engine now turns roughly 1,950 RPM, sluggish acceleration, the transmission hunts gears on hills, and the engine lugs in low RPM. To restore the original feel, you regear: target ratio = 3.73 × (35/31) = 4.21. The closest available ratio is 4.10 or 4.56, most builders pick 4.56 because aggressive off-road use favors slightly steeper ratios than the math suggests.
Rule of thumb table: 33-inch tires want 3.73 (was 3.21 stock effective), 35s want 4.10-4.56, 37s want 4.88, 40s want 5.13-5.38. Each step up costs $1,200-2,500 in parts and labor, non-trivial, which is why factoring this into a tire upgrade decision matters.
Where gear ratios show up beyond cars
- Bicycles: Gear inches = chainring teeth ÷ cog teeth × wheel diameter. A 50T chainring with 11T cog on a 700c wheel gives 121 gear inches, high speed, hard to start. 34T × 32T = 28 gear inches, climbing low gear.
- Power tools: Drill gearboxes use 50:1 or higher reductions. A motor spinning 15,000 RPM at low torque becomes 300 RPM at high torque, enough to drive lag bolts.
- Wind turbines: Blades turn 10-20 RPM. Generators need 1,500-1,800 RPM. A 100:1 gearbox bridges the gap. This gearbox is the single most failure-prone component on most turbines.
- Clocks: A second hand ticks at 1 RPM. The hour hand needs 1/720th of that. Ratios of 60:1 and 12:1 stacked together produce the slowdown.
- Watches: A mechanical movement's mainspring releases energy at ~28,800 vibrations per hour. Gear trains step that down to drive the second, minute, and hour hands at exactly the right rates.
FAQs
What is a gear ratio?
Gear ratio compares the number of teeth on two gears (or any rotating bodies). Formula: Driven gear teeth / Driver gear teeth. A 3:1 ratio means the driver turns 3 times for every 1 turn of the driven gear, slower output, more torque. Lower ratios (1.5:1) = faster output, less torque.
How does gear ratio affect car performance?
Lower (numerically higher, like 4.10:1) gear ratio = better acceleration, lower top speed, higher engine RPM at highway speed. Higher (numerically lower, like 3.08:1) = better fuel economy, higher top speed, lower highway RPM. Performance cars often use 4.10-4.56. Economy cars use 3.08-3.42.
What's the formula for RPM at a given speed?
RPM = (MPH × Gear ratio × Final drive × 336) / Tire diameter in inches. The 336 is a constant from unit conversion. Example: 65 MPH, 3.55 final drive, 1:1 4th gear, 28-inch tire: RPM = (65 × 1 × 3.55 × 336) / 28 = 2,769 RPM.
How do I find my car's gear ratio?
Several methods: (1) Look it up by VIN or owner's manual. (2) Check the differential tag (often on the rear axle housing). (3) Mark tire and driveshaft, rotate tire one full revolution, count driveshaft turns, that's your final drive ratio.
What's a good gear ratio for bigger tires?
When upsizing tires, you need a numerically HIGHER (lower in name) gear ratio to compensate. Rule of thumb: 33-inch tires need 3.73, 35s need 4.10, 37s need 4.56-4.88. Each inch of tire diameter requires ~0.10-0.15 increase in ratio for the same effective gearing.