Why do cars vary with five, six or eight speed transmissions? What is the advantage of more forward speeds? Would this affect gas mileage only? What about an electric/gas hybrid with an eight speed transmission vs. an all gas vehicle with five speeds? Not too technical an answer please! Thanks.
Gas Engines don't have a flat power curve. The transmission adapts the power curve to the road to allow the car to accelerate and cruse. Gas engines have an even narrower efficiency curve. The purpose of the extra gears is to keep the engine in its most efficient area to increase MPG. The Toyota Prius hybrid doesn't even have gears like first, second or third. It has a continuously variable transmission. The gear ratio is adjusted linearly to achieve the best engine and motor RPM's for the power demand. That's why the stock 2010-2011 Prius gets over 50 MPG.
The more gears, the closer they can be together, which means the lower the revs can stay (or the quicker you can accelerate) without a loss of top speed. If you have 5 gears vs. 8 gears with the same top speed, the 8 speed would have a much better fuel economy if driven carefully, and a much better acceleration if driven hard.
The object of more forward gears in the transmission is to take advantage of the engines maximum torque and horsepower at any speed. You aren't required to use it but it's there if you need it for towing or traveling on steep hills. Nowadays most six speed standard transmissions have two over-driven gears which would be fifth and sixth gear. On level ground while traveling at 65 MPH the engine RPM (revolutions per minute) will be very low to conserve fuel consumption.
