🚗 Gear Ratio and Top Speed

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

Working out the theoretical top speed a given gear, tire size, and final drive ratio allow at redline.

A car's top speed in any given gear is set by a simple chain of ratios: the engine spins the transmission, the transmission (through the selected gear ratio) spins the driveshaft, the final drive ratio spins the wheels, and the tire's rolling diameter converts wheel rotation into actual road speed. Multiply through that whole chain and the engine's redline RPM converts directly into a maximum speed for that specific gear.

This is why changing tire diameter or final drive ratio (a common track-day or racing setup change) directly changes top speed and acceleration character even with the exact same engine and transmission — a taller tire or a numerically lower (taller) final drive both raise top speed at the cost of acceleration, and vice versa.

Key formula

Top speed (mph) = (RPM × tire diameter (in) × π × 60) ÷ (gear ratio × final drive ratio × 63,360)

Notes & limitations

• This is a theoretical maximum assuming the engine can actually reach redline in that gear against real-world drag and available power — a car's actual top speed is often lower than this calculation if the engine runs out of power before redline at high speed, particularly in high gears.
• Tire diameter changes with wear, load, and inflation pressure in reality — this uses a single nominal diameter, which is a reasonable approximation but not an exact match to the tire's actual rolling diameter under load at speed.