Gear Ratio

Low gears have high ratios for strong acceleration; high gears have low ratios for quiet, economical cruising. The overall ratio combines the gearbox ratio with the final drive ratio. Ratios are a compromise between performance and efficiency. Modern automatics have eight or more gears so the engine can stay in its most efficient range.

A small gear driving a large one reduces speed and multiplies torque. A ratio of 3:1 means the engine turns three times for each turn of the output shaft.

Ratios affect acceleration, top speed, towing ability and fuel consumption. Changing tyre size also changes the effective overall ratio and the speedometer reading. Changing the final drive ratio is a common modification for performance or towing, but it changes speedometer calibration and fuel consumption.