Gear Ratio & Speed Calculator
Calculate vehicle speed from RPM, gear ratio, final drive, and tire size — or find RPM at a given speed. Essential for gear selection and tire sizing.
Transmission gear ratio
Overall tire diameter (not rim size)
Leave at 1.0 for 2WD or high range
How Gear Ratios Determine Speed
Your vehicle speed at any RPM depends on three things: the transmission gear ratio, the final drive (axle) ratio, and the tire diameter. Together these determine how many times the engine turns for each rotation of the wheels.
The Formula
Speed (mph) = (RPM × Tire Diameter × π) / (Gear Ratio × Final Drive × 336)
The constant 336 converts from inches-per-minute to miles-per-hour.
Common Uses
- Tire sizing: What happens to my cruising RPM if I go from 26" to 28" tires?
- Gear selection: Which final drive ratio gives me 2,000 RPM at 70 mph in overdrive?
- Top speed: What speed does my engine hit at redline in top gear?
- Towing: What RPM am I turning at highway speed in 4th gear with my current setup?
Finding Tire Diameter
If your tire is in metric format (e.g. 275/60R15), you can calculate the overall diameter: sidewall height = width × aspect ratio / 25.4, then diameter = (sidewall × 2) + rim diameter. For 275/60R15: sidewall = 275 × 0.60 / 25.4 = 6.5", diameter = 6.5 × 2 + 15 = 28".
Frequently Asked Questions
How do gear ratios affect acceleration and top speed?
A numerically higher gear ratio (e.g., 4.10 vs. 3.55) multiplies torque more, giving better acceleration but lower top speed and higher cruising RPM. A lower ratio does the opposite — less torque multiplication but higher top speed and lower RPM on the highway.
What is a final drive ratio?
The final drive ratio is the gear reduction in the differential (rear end or transaxle). It's the last stage of torque multiplication before the wheels. Common ratios range from 2.73 (highway-friendly) to 4.56 (towing/drag racing). The overall drive ratio is the transmission gear ratio multiplied by the final drive ratio.
How do I choose the right gear ratio for my vehicle?
Consider your priorities: for daily driving and fuel economy, choose a numerically lower ratio (3.08-3.55). For towing or off-road, go higher (3.73-4.56). For drag racing, 4.10-4.88 is common. Also factor in tire size — larger tires effectively lower your ratio, so taller tires often need a higher numerical gear ratio to compensate.
What RPM will my engine turn at 70 mph?
It depends on your tire diameter, transmission ratio in top gear, and final drive ratio. Use this calculator to find out. As a reference, most modern cars target 1800-2500 RPM at 70 mph in top gear for a balance of efficiency and available power.
Related Articles
How to decode tire size numbers, what happens when you change tire sizes, and how it affects speedometer accuracy, gearing, clearance, and ride quality.
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