Compute gear ratios from tooth counts, solve for RPM / speed / ratio / tire diameter, chain multiple gear stages, analyze bicycle gearing, compare transmissions gear-by-gear, and A/B test two setups — with every formula shown.
Gear Teeth → Ratio
Enter driver and driven tooth counts. Optionally add RPM and tire size to get road speed too.
Solve for RPM, Speed, Ratio, or Tire Diameter
Pick what you want to find; fill in the other three.
Multi-Stage Gear Train
Chain up to 4 gear pairs in series. Total ratio = the product of every stage — this is how big reductions are built without one giant gear.
Bicycle Gear Calculator
Chainring, cog, wheel size and cadence → ratio, gear inches, development and speed.
Per-Gear RPM / Speed Table
Pick a transmission preset (or enter custom ratios), add final drive and tire size. Enter a speed to see RPM in each gear — or flip it and enter RPM to see speed per gear.
Before / After Setup Comparison
Two gear + tire setups side by side: see exactly how RPM and speed change.
Step-by-Step Formula Trace
Every calculation above shows its work here: the formula, the numbers plugged in, and the result. This updates live as you change inputs on any tab.
Share & Saved Setups
Your inputs are encoded in the page URL — copy it to share or bookmark this exact calculation.
Worked Examples
Frequently Asked Questions
What does a 3.73:1 gear ratio actually mean?
The pinion/driveshaft rotates 3.73 times for every single rotation of the wheels. Higher numbers = “shorter” gearing: quicker acceleration and more pulling power, but higher engine RPM (and usually worse fuel economy) at a given speed.
Do numerically lower gears improve fuel economy?
Generally yes at steady highway cruise, because the engine spins slower. But go too low and the engine lugs — working harder at low RPM can actually hurt economy and drivability. Match the gear to your tire size and engine’s power band.
How does a bigger tire change my effective gear ratio?
A taller tire covers more ground per revolution, which acts like a numerically lower gear: RPM drops at a given speed, but acceleration softens. Effective ratio ≈ old ratio × (old tire ÷ new tire). Use the A vs B tab to quantify it.
What gear ratio is best for towing?
Towers usually pick numerically higher gears (e.g. 3.73 or 4.10 instead of 3.08) to keep the engine in its torque band under load. There’s no universal answer — it depends on engine torque curve, tire size, trailer weight, and terrain.
Will changing gears throw off my speedometer?
Yes — if the vehicle reads speed from the transmission output or wheel speed with a fixed calibration. After a gear or tire change, recalibrate the speedometer (tuner, speedo gear, or dealer programming depending on the vehicle).
Why use 336.13 in the RPM/speed formulas?
It’s a unit-conversion constant. One mile = 63,360 inches; an hour = 60 minutes. 63,360 ÷ (π × 60) = 336.13 — it folds the inches-to-miles and minutes-to-hours conversions into a single number.
How do I count gear teeth without taking the axle apart?
First check the axle tag bolted to the differential cover or the RPO/service-parts sticker in the glove box — many list the tooth counts (e.g. “41/11”) or the ratio directly. If the tag is gone, jack up one wheel, mark the driveshaft and the tire, rotate the tire exactly two full turns, and count driveshaft rotations: with an open differential and one wheel held, the driveshaft turns equal twice the ratio. A 3.73 axle shows about 7.5 driveshaft turns.
Why do two calculators give slightly different answers for the same inputs?
Usually rounding. One calculator may use 3.73 while another uses the exact tooth-count ratio (41 ÷ 11 = 3.727). That small difference shifts the speed result by a fraction of a mph. Also check that both calculators use the same tire diameter and the same transmission gear assumption.
What is the difference between gear ratio and transmission gear?
The gear ratio here usually means the final drive (axle) ratio — fixed for your vehicle. Transmission gears change every time you shift: each gear multiplies the final drive. Final drive ratio = transmission gear ratio × axle ratio. Cruising RPM depends on the final drive; the number on your axle tag is only half the story.
How does gear ratio affect towing?
A numerically higher ratio (e.g. 4.10 vs 3.42) multiplies engine torque more, so the truck pulls harder from a stop and holds speed on grades — at the cost of higher cruising RPM and more fuel burn. That is why towing packages often come with higher-ratio axles. Calculate your highway RPM with both ratios to see the trade-off in concrete numbers.
Does changing the gear ratio affect the odometer?
Yes. The odometer is calibrated to a specific final drive ratio and tire size. Changing the axle ratio (or tire size) without recalibrating makes the odometer and speedometer read wrong — typically by the same percentage as the ratio change. A tuner or speedometer calibrator corrects this after a gear swap.
Limitations
Gear Ratio Calculator
A gear ratio calculator tells you how your drivetrain multiplies engine torque and how it affects speed. Enter the tooth counts on your gears — or enter your engine RPM, tire size, and axle ratio — and it gives you the ratio plus the real-world speed or RPM numbers that follow from it.
Use it before buying gears for a truck, bike, or go-kart, or to check whether your current setup matches the speedometer reading you expect.
How to use this gear ratio calculator
The tool has two modes. Use whichever one matches the numbers you have.
Mode 1: Gear-teeth mode (from tooth counts)
- Count the teeth on the driver gear (the gear connected to the engine or input). For an axle, this is the pinion gear.
- Count the teeth on the driven gear (the gear connected to the wheels or output). For an axle, this is the ring gear.
- Enter both counts. The calculator divides driven teeth by driver teeth and returns the ratio, e.g. 3.73:1.
- Read the result: a 3.73:1 ratio means the driver turns 3.73 times for every single turn of the driven gear — engine torque is multiplied roughly 3.73×.
Mode 2: RPM / tire / speed mode (from driving numbers)
- Enter engine RPM — for example, the RPM your truck holds at highway cruising.
- Enter tire diameter in inches (measure it, or calculate it from your tire size code).
- Enter the axle (final drive) ratio — check the door jamb sticker, the axle tag, or the owner’s manual.
- The calculator returns your road speed at that RPM. You can also solve in reverse: enter speed and RPM to back out the ratio, or enter speed and ratio to find the RPM.
Formulas with worked examples
Gear ratio from tooth counts
Gear ratio = driven teeth ÷ driver teeth
Worked example: a ring gear with 41 teeth and a pinion with 11 teeth.
41 ÷ 11 = 3.727… → 3.73:1
That is the ratio printed on the axle tag as “3.73”. Every full turn of the wheels takes 3.73 turns of the driveshaft.
Road speed from RPM, tire size, and ratio
mph = RPM × tire diameter (inches) ÷ (gear ratio × 336.13)
Worked example: 2,500 RPM, 28-inch tires, 3.73 axle ratio.
2,500 × 28 = 70,000
3.73 × 336.13 = 1,253.76
70,000 ÷ 1,253.76 = 55.8 mph
The constant 336.13 combines the minutes-per-hour, inches-per-mile, and π conversions so the units work out to miles per hour. If you use a transmission gear other than 1:1, multiply the axle ratio by the transmission gear ratio first (final drive = transmission gear × axle ratio) and use that number in place of “gear ratio”.
Common mistakes
- Mixing up driver and driven. The ratio is driven ÷ driver. Reversing them gives you the inverse (0.27 instead of 3.73) and everything downstream is wrong.
- Using millimeters for tire diameter. The 336.13 constant expects inches. If your tire data is in millimeters, convert first (1 inch = 25.4 mm).
- Confusing the transmission gear ratio with the final drive ratio. A “3.73 axle” with a 0.67 overdrive gear gives a final drive of 3.73 × 0.67 = 2.50. For speed calculations, always use the combined final drive, not the axle number alone.
- Ignoring tire growth and wear. Advertised tire sizes are nominal — a worn tire is effectively smaller, and some tires grow slightly at speed. The result is accurate to roughly ±2%, which is fine for planning but not a replacement for a measured speedometer check.
- Forgetting which gear you’re in. RPM-to-speed math only holds for the gear you calculated for. The same 2,500 RPM means a very different speed in 3rd gear versus 5th.