Gear Ratio Calculator
Calculate speed from RPM, find RPM needed for speed, or compute gear ratio from tooth counts. Works with any transmission and differential setup.
What do you want to calculate?
Engine revolutions per minute
Transmission gear ratio (engine revs / wheel revs)
Overall tire diameter in inches
Differential gear ratio (typically 3.08-4.56)
Vehicle speed in miles per hour
Number of teeth on input gear
Number of teeth on output gear
What is a gear ratio?
Gear ratio is the ratio of input revolutions to output revolutions. A 3.5:1 ratio means the input gear turns 3.5 times for every 1 turn of the output gear. Lower ratios (like 2:1) give more speed but less torque. Higher ratios (like 4:1) give more torque but less speed.
How does gear ratio affect speed?
Higher gear ratios reduce wheel RPM for a given engine RPM, which means lower vehicle speed. The transmission multiplies engine torque - first gear might be 4:1 (high torque for starting), while fifth gear might be 0.8:1 (overdrive for highway cruising). This is why you rev high in first gear but low at highway speed.
What is the difference between transmission ratio and rear end ratio?
The transmission ratio varies by which gear you're in - first gear might be 4:1, fifth gear might be 0.85:1. The rear end (differential) ratio is fixed - typically 3.08 to 4.56 in passenger cars. Multiply them together for total reduction. A Mustang with 3.73 rear end and 2.66 first gear has 3.73 × 2.66 = 9.93:1 total reduction.
How do I calculate speed from RPM and gear ratio?
Formula: Speed = (RPM × Tire Circumference) / (Gear Ratio × Rear End × 63360). Where 63360 converts inches/minute to miles/minute. Example: 3000 RPM, 3.5 gear, 3.73 rear, 26" tires: Speed = (3000 × 81.68) / (13.06 × 63360) = 29.7 mph in that gear.