Diff Gear Ratio Calculator

Calculate diff ratio from tire size, RPM, and speed.

Changed tire sizes and need to know what diff ratio keeps your RPM in the sweet spot? Diff Gear Ratio Calculator finds the required differential ratio for any tire, speed, and RPM combination — so you pick the right ring and pinion before you buy.

Enter tire dimensions (width, aspect ratio, wheel diameter), engine RPM, vehicle speed, and transmission ratio. Get tire diameter, circumference, revolutions per mile, and the exact diff ratio you need.

WHAT IT CALCULATES:

• Overall tire diameter from metric tire size
• Tire circumference in inches
• Revolutions per mile
• Required differential ratio for your RPM and speed target
• Actual speed at a given RPM (with known diff ratio)
• Actual RPM at a given speed (with known diff ratio)

DRIVETRAIN INPUTS:

• Engine RPM
• Vehicle speed (mph)
• Transmission gear ratio
• Transfer case ratio (for 4WD vehicles)

TIRE SIZE INPUTS:

• Tire width (mm)
• Aspect ratio (%)
• Wheel diameter (inches)

OPTIONAL KNOWN DIFF RATIO: Enter your current or planned diff ratio to see speed-at-RPM and RPM-at-speed calculations. Compare before and after a regear or tire swap.

Built for automotive enthusiasts, 4x4 builders, and drivetrain shops. Works completely offline — no internet required.