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 = 58,447 ÷ 26 = 2,248 RPM

That is a typical highway cruising speed for the engine, low enough to be quiet and efficient.
In 1st gear at the same road speed: RPM = (70 × 3.8 × 3.55 × 336) ÷ 26 = 12,203 RPM, which is roughly double any production redline. That is the whole reason a gearbox has more than one gear.

A note on redlines. Most naturally aspirated petrol engines cut fuel somewhere between 6,000 and 7,000 RPM, turbo engines a little lower, and diesels a lot lower, often 4,500. Motorcycles and high-strung sports engines go well past that. This calculator flags anything over 7,000 as suspicious, but check your own tachometer before treating the flag as gospel.

“Gear ratio” means the opposite thing on a bicycle, and that is not a mistake on either side. Here a ratio of 3.55 means the engine turns 3.55 times for one turn of the wheel, so a bigger number is a lower gear. On a bike, a ratio of 3.55 means the wheel turns 3.55 times for one turn of the pedals, so a bigger number is a higher gear.
Both fields write it as driving over driven from their own end of the drivetrain, and both have written it that way for a century. If you have landed here wanting chainrings and cassettes, the cycling gear ratio calculator is the page. Clockmakers use the automotive convention, wheel over pinion, on the clock gear train ratio calculator.