Gear Ratio / RPM at Speed Calculator

Gear ratio defines the mechanical relationship between the engine’s rotation speed (RPM) and the wheel/output shaft rotation. It determines how torque is multiplied (or reduced) and how many engine revolutions are needed per wheel revolution.

Gear ratio formula: Gear Ratio = Driven Gear Teeth ÷ Driving Gear Teeth

Or equivalently: Gear Ratio = Input RPM ÷ Output RPM

Engine RPM at a given vehicle speed: Engine RPM = Vehicle Speed × Gear Ratio × Final Drive Ratio × (63,360 ÷ (Tire Circumference in inches × 60))

Simplified: RPM = (Speed in mph × Gear Ratio × Final Drive Ratio × 336) ÷ Tire Diameter (inches)

Where:

• Gear Ratio — the current transmission gear’s ratio (e.g. 1st gear ≈ 3.5:1, 6th gear ≈ 0.75:1)
• Final Drive Ratio — the rear differential or front drive axle ratio (typically 2.73:1 to 4.10:1)
• Tire Diameter — measured from the tire size code (e.g. 225/45R17 has diameter ≈ 24.97 inches)
• 336 — a conversion constant combining unit conversions (mph → inches/minute → RPM)

Typical transmission gear ratios:

• 1st gear: 3.0–4.5:1 (high torque multiplication for starting)
• 2nd gear: 1.8–2.5:1
• 3rd gear: 1.2–1.7:1
• 4th gear: 0.9–1.1:1 (often near 1:1 = direct drive)
• 5th gear: 0.7–0.9:1 (overdrive)
• 6th gear: 0.6–0.8:1 (overdrive, fuel economy)

Worked example: Car in 6th gear (ratio 0.70:1), final drive ratio 3.55:1, tire diameter 26 inches, traveling at 70 mph.

RPM = (70 × 0.70 × 3.55 × 336) ÷ 26 = (70 × 0.70 × 3.55 × 336) ÷ 26 = 58,408 ÷ 26 = 2,246 RPM

This is a typical highway cruising RPM — low enough for fuel efficiency. In 1st gear at the same speed: RPM = (70 × 3.8 × 3.55 × 336) ÷ 26 = 12,192 RPM — well beyond redline, which is why higher gears exist.