Gear Ratio Calculator
Find your road speed at any engine RPM, or the RPM you'll turn at a target speed, from your gear ratio, final drive, and tire diameter.
Bottom line: road speed per RPM is set by three things — the gear you are in, the final drive (axle) ratio, and your tire diameter. Taller tires and lower final-drive numbers raise speed-per-RPM and drop cruising revs; shorter gears do the opposite and quicken acceleration. This calculator does both directions instantly — then verify the real number with GPS.
How to Use the Calculator
- Enter your transmission gear ratio (1.00 = direct top gear; overdrive gears are below 1.00)
- Enter your final drive ratio (also called the differential or axle ratio)
- Enter your tire diameter in inches — not sure? Use the Tire Size Calculator
- Choose a mode: enter an engine RPM to get road speed, or a target speed to get the RPM
The Formula
Road speed is set by how far the tire rolls per revolution and how many engine turns it takes to spin the tire once through the gearbox and axle:
- Speed (mph) = (RPM × Tire Diameter × π × 60) ÷ (Gear Ratio × Final Drive × 63360)
- RPM = (Speed × Gear Ratio × Final Drive × 63360) ÷ (Tire Diameter × π × 60)
63,360 is the number of inches in a mile; π × Tire Diameter is the tire's circumference in inches; the 60 converts minutes to hours.
What Each Input Does
- Final drive: the differential / axle ratio. It multiplies the gearbox ratio, so it scales every gear at once.
- Taller tires (larger diameter) raise speed per RPM and lower your effective gearing — quieter cruising, softer acceleration — and make the speedometer read low.
- Shorter gears (a numerically higher final drive) improve acceleration by multiplying wheel torque, but raise cruising RPM and hurt economy and top speed.
- Gear ratio is the specific transmission gear you are in; 1.00 is a direct-drive top gear, while overdrive top gears are below 1.00.
Common Final-Drive Ratios (Approximate)
Ranges are approximate and vary by chassis, tire size, and gearbox spread.
| Final Drive Range | Character | Best For |
|---|---|---|
| 2.73 – 3.08 | Highway / economy | Low cruising RPM, quiet, fuel-efficient long-haul |
| 3.31 – 3.73 | Balanced | Everyday street cars — decent launch, reasonable cruise |
| 3.90 – 4.30 | Drag / acceleration | Quick launches and roll races; higher cruising RPM |
| 4.30+ | Track / strip | Maximum acceleration, short gearing, limited top speed |
Theoretical vs. Real — Verify with GPS
This math assumes exact ratios, zero driveline slip, and your true rolling tire diameter. Real speed can differ if the speedometer is off or the tire is worn. FastTrack measures true road speed with GPS Doppler fused to a 100Hz accelerometer, so you can compare the RPM-based prediction against your actual speed and catch a mis-read speedometer or the wrong tire diameter — for free.
Download FastTrack Free on the App Store
Tire Size Calculator | Quarter Mile Calculator | All Racing Calculators | GPS Timing
Frequently Asked Questions
How do I find my final drive ratio?
The final drive (also called the differential or axle ratio) is usually listed in your owner's manual, on a tag on the differential housing, or in the door-jamb / build sheet code. Common values are 2.73, 3.08, 3.55, 3.73, and 4.10. On many cars you can also decode it from the axle code or look it up by trim and drivetrain. If in doubt, jack up one wheel, mark the driveshaft and tire, and count driveshaft turns per one tire revolution.
How do taller tires change my gearing and speedometer?
A taller (larger diameter) tire travels farther per revolution, so it raises road speed per RPM and effectively lowers your gearing — cruising RPM drops, but acceleration softens because the driveline has less mechanical advantage. It also makes the speedometer read low: if the car thinks the tire is the stock size, your true speed is higher than the display. Shorter tires do the opposite. Enter your actual tire diameter here to see the real numbers.
What RPM will I cruise at on the highway?
Switch this tool to RPM at Speed, enter your top-gear ratio (often around 0.65 to 1.00 on an overdrive gearbox), your final drive, and your tire diameter, then enter your cruising speed (say 70 mph). The result is the engine RPM you will hold at that speed. Lower final-drive numbers and taller tires give lower, quieter cruising RPM.
Do shorter gears make my car faster?
Shorter gears (a numerically higher final drive, like going from 3.55 to 4.10) multiply torque at the wheels, so the car accelerates harder and quicker to a given speed. The trade-off is higher cruising RPM, more noise, worse fuel economy, and a lower top speed for a given redline. They help most in the quarter mile and roll races; they hurt on the highway.
How do I verify my real top speed or speed at a given RPM?
A calculator assumes zero tire slip, exact ratios, and a perfect tire diameter. To confirm reality, use FastTrack: it measures true road speed with GPS Doppler fused to a 100Hz accelerometer, so you can compare the RPM-based prediction against your actual speed and catch a mis-read speedometer or the wrong tire size. FastTrack is free.