Why do cars with automatic transmissions have tachometers 2607

Ummmmm, no. The most fuel-efficient RPM of an Otto Cycle engine is well below the RPM that will produce the most horsepower. "Lugging" the engine is not fuel-efficient, as the engine RPM is too low. At low RPM, too much energy is wasted as heat. (a lesser percentage of energy is converted to kinetic energy while the engine is lugging) Overdrive is meant to improve fuel efficiency at higher speeds. (OVER drive speeds) It can't do this by "lugging" the engine. It does this by reducing the RPM into a more fuel-efficient range while still NOT lugging it. Good example: My car is at 3500RPM-43%(redline) and .6 (of max horsepower RPM) at 78MPH in OVERdrive, where it easily gets 42MPG. That's just the way it was designed. If I downshift it to drive, the most fuel-efficient 3500RPM-43%(redline)-.6(of max horsepower RPM) equates to exactly 55MPH. I have no idea how fuel-efficient it is at 55MPH in drive, as there is nowhere near me where I could safely test that. Even the highways posted at 55MPH have average traffic speeds well in excess of 70MPH, and it's just not a good idea to hold a steady speed 20MPH below the flow in any lane. BUT, I suspect my vehicle would be quite fuel-efficient at 55MPH, as that is how it was designed. The most fuel-efficient RPM of my engine happens to fall right AT 55MPH, when the car is in drive gear. No way was THAT a coincidence, so it had to be by design.

Yet since I learned how to find the most fuel-efficient RPM of an engine, I have discovered that a lot of vehicles are geared incorrectly. My wife's car is one of them. Her car should be most fuel-efficient at ~ 3400RPM, yet it is hard to get her engine to even hit 3000 (without driving fast enough to lose your license), which is the point of maximum torque. A lot of cars I've driven have been similarly geared incorrectly. In one extreme example, I drove a chevy with a 4-cylinder engine that rarely hit 2000RPM. It was horrible, as far as fuel economy goes. Now I know why.

If "lugging" the engine was such a good idea, I'd expect to see more 7 or 8-speed trannies, both automatic and manual. That is, there should be more "over"drive gears to keep the engine running really slow. But if the car is geared correctly, only one overdrive gear is needed. This allows the car engine to max out the EPA estimate at 55MPH (where it is tested, in drive gear), and yet still get decent fuel economy closer to speeds that the car is actually driven. Thus, more overdrive gears would be redundant on a street-legal car. -Dave