Part of the Automotive collection — 4 tools available
Gear Ratio Calculator — Speed Per Gear
Calculate vehicle speed in each gear at a given RPM. Plan gear ratio changes and see how transmission ratios affect acceleration and top speed.
About This Calculator
Your transmission gear ratios determine how engine power reaches the wheels, affecting everything from launch acceleration to highway cruising RPM and fuel economy. Close-ratio transmissions keep the engine in its peak power band between shifts for maximum performance, while wide-ratio setups offer a balance of strong acceleration off the line and relaxed highway cruising. Changing tire size also affects your effective final drive ratio, often by a surprising amount. Our gear ratio calculator shows your speed in every gear at any RPM so you can plan ratio changes or compare transmissions.
The Formula Behind This Calculator
Ratio = Driven teeth / Drive teeth Final = Trans gear * Diff ratio RPM = (Speed * Final * 336) / Tire dia.
Understanding the math helps you verify results and make better decisions for your project.
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How to Use
- 1Enter the engine RPM you want to analyze (redline for max speed per gear).
- 2Input each gear ratio from your transmission specs.
- 3Set the final drive (differential) ratio.
- 4Enter your tire diameter to complete the calculation.
When to Use
- →Choosing between different transmission or differential options.
- →Planning a gear ratio swap for drag racing or track days.
- →Understanding how tire size changes affect each gear's speed range.
Tips
- ✓For drag racing, you want each gear to drop RPM to the start of your power band after shifting.
- ✓A numerically higher final drive ratio (e.g., 4.10 vs 3.55) gives quicker acceleration but lower top speed and higher highway RPM.
- ✓Track cars benefit from close-ratio gearboxes; street cars prefer wider ratios for flexibility.
FAQ
What does a close-ratio transmission mean?
A close-ratio transmission has small steps between gears (e.g., 2.0, 1.6, 1.3, 1.1, 0.9). This keeps the engine in its optimal RPM range but sacrifices low-speed launch torque.
Should I change my final drive or my transmission?
Changing the final drive is cheaper and affects all gears proportionally. Changing the transmission lets you tune individual gear ratios but costs significantly more.
What RPM drop should I expect between shifts?
With typical gear spacing, expect a 1500-2500 RPM drop between shifts. For a 6,500 RPM redline, the engine should land above 4,000 RPM after each upshift for best acceleration.
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