Gear Ratio / RPM at Speed Calculator

Engine RPM at a given speed is determined by tire size, final drive (differential) ratio, and transmission gear ratio.

Imperial formula: RPM = (Speed (mph) x Final Drive x Gear Ratio x 336) / Tire Diameter (inches)

The constant 336 accounts for the conversion between mph and tire revolutions per minute.

Metric formula: RPM = (Speed (km/h) x Final Drive x Gear Ratio x 5305.1) / Tire Diameter (mm)

Common final drive ratios:

• Economy cars: 3.07–3.55
• Sport sedans: 3.73–4.10
• Trucks: 3.55–4.56

Common transmission ratios (manual):

• 1st gear: 3.2–3.8
• 2nd gear: 1.8–2.2
• 3rd gear: 1.3–1.5
• 4th gear: 1.0 (direct drive)
• 5th gear: 0.75–0.85 (overdrive)
• 6th gear: 0.62–0.73 (overdrive)

Tire diameter quick reference:

• 205/55R16: 24.9 in (632 mm)
• 225/45R17: 25.0 in (634 mm)
• 265/70R17: 31.6 in (803 mm)
• 275/60R20: 33.0 in (838 mm)