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This question has bugged me for years Why do vehicles...

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Dear Tom and Ray:


This question has bugged me for years. Why do vehicles with automatic transmissions have mileage ratings that are lower than vehicles with manual transmissions? My father says it's because cars with automatic transmissions have to idle higher. That could explain the difference in city driving, but that shouldn't make a difference on the highway, should it? Does it have to do with added weight? Please help. I don't want to go to my grave not knowing.
Robert

TOM: I hope you're young, Robert, because this is a long answer.

RAY: To put it simply, both types of transmissions connect the car's engine (which produces the power) to the driven wheels (which use the power). Other things being equal, the less power you "lose" or "waste" along the way, the higher your mileage is going to be.

TOM: Manual transmissions connect the engine and the wheels with a dry clutch, which makes a solid, mechanical connection between the two. So theoretically, once you're in gear, no energy is being wasted.

RAY: Automatic transmissions, on the other hand, use a FLUID coupling called a "torque converter." The engine moves a fluid, and that fluid, in turn, makes the wheels move. And you DO lose some power in that transition.

TOM: Imagine that you had two electric fans facing each other. One of them is plugged in, and the other one isn't. And they're both sealed in a big container of maple syrup. You turn on the fan that's plugged in, and it spins and sets the maple syrup in motion. Eventually, the motion of the maple syrup will make the other fan start to turn, right?

RAY: And that's essentially how the power is transmitted from the engine (the electric fan that's plugged in) to the rest of the transmission and the wheels (the fan that's not plugged in).

TOM: And as you can imagine,
some of the energy is lost, because the fluid isn't as efficient as a direct, mechanical coupling. That loss of energy is called slippage, and because an automatic transmission is always slipping a little bit, the mileage it gets is not as good as a manual transmission.

RAY: But things are changing. In fact, some automatic transmissions now get just as good mileage as manuals, and a few get even BETTER mileage.

TOM: How come? Well, automatic transmissions now combine the best of both worlds. At lower speeds they work like the fans in the maple syrup we just described. But at higher speeds, over 40 miles per hour or so, most of them have a thing called a "lock up torque converter," which--as the name implies--locks up, and makes a solid connection, just like a manual transmission. So at highway speed--when the greatest amount of fuel gets used--modern, automatic transmissions are very efficient.

RAY: In addition, automatics often use bigger ratio overdrive gears than manuals, which allows the engine to turn even slower at highway speed, further increasing your miles per gallon.

TOM: So there's no longer any good economic argument for driving a standard transmission...especially when you factor in the inevitable clutch job or two you'll need. It's purely a matter of preference these days. If you like shifting, then get a stick shift.

RAY: But if you have better things to do with your right hand and left foot while you drive, get an automatic, and don't worry about the mileage difference.

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