Gearing & tire size

Results update immediately as inputs change. Enter focuses the first invalid field; Reset restores the defaults. Compare one gear at a time. For a bicycle, enter integer tooth counts and pedal cadence. Wheel RPM = cadence × chainring / sprocket; travel per crank turn = circumference × chainring / sprocket. Gear inches express the equivalent direct-drive wheel diameter in inches. Select Input RPM and enter a target speed to find the required cadence or engine RPM in the selected gear.

For a car or motorcycle, enter engine RPM and reduction ratios as input turns per output turn. Wheel RPM = engine RPM / (gear × final drive × primary). Primary defaults to 1; for a motorcycle with a primary reduction, enter it in the expandable menu. A ratio below 1 represents overdrive. This is kinematics, not a prediction of attainable top speed: power, drag, slip and tire growth are not modeled. Bicycle hub gears and motor-assisted cadence are not modeled.

For best accuracy measure one loaded wheel revolution on the ground, with normal pressure and rider/load, and enter its distance in mm. Speed in km/h = wheel RPM × circumference in mm × 60 / 1,000,000. Zero cadence/RPM gives zero speed. Diameter mode uses circumference = π × diameter. ETRTO 37-622 specifies width and bead-seat diameter, not outside diameter: the optional rough estimate assumes tire height equals width, giving 622 + 2 × 37 = 696 mm. 700C is not a measured 700 mm outside diameter.

Metric radial code 205/55 R16 gives nominal diameter 16 × 25.4 + 2 × 205 × 0.55 = 631.9 mm. Markings such as 91V, (97Y) and XL are skipped when calculating dimensions. Dimensions are geometric estimates, not fitment approval or speedometer calibration. See Schwalbe: tire dimensions, Continental: tire codes and TREMEC: gear ratios.