Gear Ratio & Speed Calculator
Enter transmission gear, final drive ratio, tire diameter, and RPM.
Affiliate links — we may earn a commission at no cost to you. Learn more
Get your results + weekly automotive tips
One email per week. Tips, tool deals, new calculators. Unsubscribe anytime.
Free tools, useful tips. We recommend products we'd actually use.
Calculating Speed from Gear Ratios
Vehicle speed depends on engine RPM, transmission gear ratio, final drive (axle) ratio, and tire diameter. Higher RPM and taller gears (lower numerical ratio) produce higher speeds.
The formula: MPH = (RPM × tire diameter × π) / (overall ratio × 336 × 12). This calculator simplifies it for instant results.
Gear ratio = number of teeth on driven gear / number of teeth on driving gear. A 3.73 rear axle ratio means the driveshaft turns 3.73 times for each wheel revolution. Higher axle ratios (numerically larger, e.g., 4.10) give more torque at the wheels but lower top speed; lower ratios (e.g., 3.08) give better fuel economy and higher top speed.
Overall drive ratio = transmission gear ratio × axle ratio. In 1st gear (3.36:1 typical) with a 3.73 rear end: overall ratio = 3.36 × 3.73 = 12.53:1. The engine turns 12.53 times for each wheel revolution — maximum mechanical advantage for acceleration from a stop.
Speed at RPM calculation: vehicle speed (mph) = (RPM × tire circumference in inches) / (overall gear ratio × 1056). At 3,000 RPM in 5th gear (0.84:1) with 3.73 axle and 28-inch diameter tires: tire circumference = π × 28 = 87.96 inches. Speed = (3000 × 87.96) / (0.84 × 3.73 × 1056) = 263,880 / 3,308 = 79.8 mph.
Highway cruising RPM: lower numerical axle ratios (or taller overdrive gears) lower RPM at highway speed, improving fuel economy. Dropping from a 3.73 to a 3.08 rear end reduces highway RPM by approximately 17% — significant for V8 trucks that cruise at 2,200 RPM vs. 1,800 RPM.
See also: HP & Torque Calculator · Quarter-Mile Calculator