Fact Sheet: Wider Is Better
Source: Ford Motor Company Press Release
Ever try to get a vehicle moving from a dead stop with the manual transmission shifted into high gear? It doesn't work very well, because the engine doesn't have enough torque at low speed.
That's where gear ratios come in. The gears inside a transmission offer a way to multiply engine torque when needed - such as at low speed, or when the vehicle is heavily loaded.
The six-speed transmission in the all-new 2005 Ford Five Hundred, for example, offers a first gear with more than four teeth for every tooth on the shaft coming from the engine. As those two gears mesh together, the transmission multiples engine torque at a rate of 4.41:1 to get the vehicle moving smoothly from a stop. The tradeoff is speed - in first gear the engine turns 4.41 times faster than the drive shafts, so even if the engine reaches top speed, the vehicle won't move very fast.
Modern automatic transmissions such as the Five Hundred's six-speed constantly balance engine speed, vehicle speed and the need for acceleration. It smoothly shifts from first gear to second - which has a ratio of 2.370:1 - and then to third and so on as the vehicle accelerates.
Overdrive is the term for gear ratios that are lower than 1:1 - a situation where the engine is turning slower than the drive shaft. Overdrive gears are appropriate when very little torque is needed - the vehicle is no longer accelerating - and can deliver the best fuel economy. The six-speed transmission in the Five Hundred has two overdrive gears, with a final ratio of 0.686:1.
Ratio span is the mathematical difference between the highest top gear and the lowest low gear. A wide span offers both a strong low gear for acceleration and an economical high gear for highway cruising.
The new CVT and 6-speed transmissions that Ford is developing allow a 6:1 ratio span - compared with about 4:1 in a 4-speed transmission. The result is strong acceleration from a dead stop, and good fuel economy.