Final Drive Ratio

The final drive ratio is the last stage of gear reduction in a vehicle's drivetrain, applied between the output of the transmission and the road wheels. It exists because the gearbox alone usually cannot provide enough overall reduction to match a fast-spinning engine to comparatively slow-turning wheels. A permanent, fixed reduction at the end of the chain multiplies torque one final time and brings the rotational speed down to a usable range, so the figures chosen for it have a pervasive effect on how the whole car performs.

Physically this reduction takes place within the differential, or within the transaxle on front-wheel-drive and many transverse layouts where the gearbox and final drive share a single casing. It is created by the ratio between a small pinion gear driven by the transmission and a much larger crown wheel, or ring gear, attached to the differential. A final drive ratio of, say, 3.9 to 1 means the crown wheel has 3.9 times as many teeth as the pinion, so the input must turn 3.9 times for each turn of the differential. On rear-drive cars this pinion and crown wheel meeting at right angles also turn the drive through ninety degrees from the propeller shaft to the half-shafts.

The significance of the ratio comes from the fact that it multiplies with every gear in the transmission. The overall ratio in any given gear is the gearbox ratio for that gear multiplied by the final drive ratio, and it is this combined figure that determines wheel torque and the road speed produced at a particular engine speed. Because the final drive applies to all gears alike, changing it shifts the character of every ratio at once, which makes it a powerful tuning tool.

This is where the balancing act lies. A numerically higher, shorter final drive multiplies torque more strongly, sharpening acceleration and improving the ability to pull away or climb, but it raises engine speed at any given road speed and so harms fuel economy and increases noise when cruising. A numerically lower, taller final drive does the opposite, lowering motorway revs and saving fuel at the expense of outright pull. Manufacturers select the figure to suit a model's intended use, and enthusiasts sometimes change the differential gears to retune a car.

In practical terms the final drive ratio works hand in hand with the overdrive top gears of modern transmissions, which let designers fit a fairly short final drive for brisk lower gears while still keeping cruising revs low. It is closely related to the axle ratio, often the same thing expressed differently, and to the individual gear ratios it multiplies, all housed within or fed by the differential. Understanding it explains why two otherwise identical cars with different rear-axle or transaxle ratios can feel markedly different to drive.