How does an automatic transmission get better gas mileage than a manual?

Technology is getting better for the automatic transmission, but new specs are supposed to be more true world for mileage expectations. The truth is, a lot of people really don't know how to drive a stick for economy, they think stick is for better performance. You can't have your cake and eat it too. Give me 2 cars, one auto, one stick, with the same number of speeds, and same final drive ratio, and I can get fractionally better mileage out of the stick every time, by driving for economy. However, if you have a 4-speed stick, and a 5 or 6-speed auto, the auto will do better. Sometimes, comparing the stick to the auto is like comparing apples to oranges.

Manuals do (if properly driven). Automatic transmissions rely on a fluid coupler to connect the engine to the driveshaft. There is always some sort of slippage resulting in wasted energy. A manual transmission is directly connected via a clutch which offers no slippage unless it is defective. It's the fluid coupling (called a "torque converter") that is the culprit in most automatic transmissions disadvantage in fuel mileage. There is slippage when an automatic changes gears. Once you are under way, most modern automatics will lock the torque converter so that there is no slippage, but the control computer is usually programmed so that the slightest grade causes it to unlock, and in some conditions, to downshift. All else being equal, staying in the highest gear practical gives you the highest fuel mileage. There are two main reasons. -Friction is more a factor the faster the engine is running. Keeping it in a high gear lowers rpm and thus friction. -Your internal combustion engine is an air pump. It sucks in air to mix with fuel to burn, and then exhales the products of that burning. A higher gear requires a bigger throttle opening to maintain speed, which is less restrictive. Let me 'splain The larger the opening letting air into your engine, the less work it has to do to suck it in. Try breathing just through a soda straw if you want a demonstration. It might seem that a wider throttle opening would burn more gas, but it does not. At the same speed you burn the same amount of fuel to overcome the drag of the wind, friction in the drive train, and rolling resistance of the tires. The only other factors are internal friction in the engine, and intake restriction, both of which are optimized in the highest practical gear for conditions. A manual transmission gives you absolute control over what gear you are in.

Technology is just getting better. Autos used to be 3 speeds or maybe 4 speeds. Having fewer forward gears meant having to accelerate more before being able to shift into the next gear. But now with 5 and 6 speeds becoming common, coupled with smoother faster shifting, they are able to get better mileage.

That's not true. Automatic tranny gets about 10% hotter. So that heat (energy) has to come from somewhere.

The only exception might be (MIGHT) are some "automatic" tranny with REAL clutch (new Mustangs). Then they are really manual with computer controlled clutch. Then the computer can be programmed to shift at the most efficient point.

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But that is NOT to say that in REAL life (not factory estimate), that a manual car will always get better MPG than a auto. Most people who shift late will most likely get worse MPG than in an auto.

But I think you were asking about the FACTORY ESTIMATE.

Good Luck...

The standards use backpressure that helps to slow down, but the automatics freewheel when you let of the gas, sort of like putting it in neutral and coasting...plus they use big overdrive ratios..