Enter your Engine RPM, Tire Diameter, Transmission Gear Ratio, and Differential Ratio (Ring & Pinion) into the Engine RPM to Speed Calculator to find your Vehicle Speed, along with Wheel RPM and your speed in km/h — useful any time you're swapping gears, changing tire sizes, or re-gearing your diff.
Vehicle Speed
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Wheel RPM
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Speed (km/h)
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RPM stands for Revolutions Per Minute, measuring how fast your engine crankshaft rotates. Vehicle speed depends on engine RPM, transmission gear ratio, differential ratio, and tire size working together through the drivetrain.
Check your owner's manual or transmission specifications. Common ratios include 1.0:1 for direct drive, 0.7:1 for overdrive, and higher ratios like 2.5:1 to 4.0:1 for lower gears. Automatic transmissions typically use 0.7:1 in top gear.
Differential ratio (ring and pinion gear ratio) is found on a metal tag on your rear axle differential housing. Common ratios range from 2.73:1 to 4.56:1. Higher numbers provide more torque but lower top speed.
Measure from ground to top of tire when properly inflated and loaded. You can also calculate it from tire size markings (e.g., 265/70R17) or use online tire size calculators for precise diameter in inches.
Speedometers can have 2-5% error, tire pressure affects diameter, and tire wear changes the effective diameter. Also ensure you're using the correct gear ratio for the transmission gear you're currently in.
Larger tires increase speed at the same RPM because they cover more ground per revolution. A 1-inch diameter increase typically adds about 3-4% to your speed at any given engine RPM.
Yes, this calculator works for any vehicle with the same drivetrain principles. Just ensure you use the correct transmission ratio, differential ratio (if applicable), and tire diameter for your specific vehicle.