It's been common knowledge for years that automatic transmissions cost you about 5% on gas mileage compared to a stick shift. T'other day I was reading an internet rant that claimed the "modern" slushbox was in fact just as efficient as manual shift. I beg to differ.
Car transmissions have a simple problem to deal with. When stopped at a light, the wheels must not turn and the engine must turn. Automatic transmissions use a fluid coupling between engine and gears to solve this problem. The engine turns a paddle wheel gizmo in a can of hydraulic fluid and a similar gizmo on the gear shaft gets swirled around and around by the fluid. When stopped at a light, the brakes hold the car stopped, and the fluid just gets churned up. Trouble is, when the car is moving, the fluid coupling or "torque converter" is still there, and there is some slip between the engine in the gears. About 5%, which accounts for the 5% loss of gas mileage.
Some advanced automatic transmissions a have an additional lockup clutch which grabs the two paddlewheel gizmos together when the car is moving and prevents any slippage. Transmissions with a lockup clutch are pretty good, but not all cars have them. For instance Cadillac has a lockup clutch and enjoys significantly better gas mileage than the big Ford/Mercury/Lincoln sedans which don't.
So, when asked what you have done for the environment lately, you can respond by asking them if they drive a stick shift. And when buying a new car, you can mystify the salesman by asking if the car has a transmission lockup clutch. The device will save you 5% on gas mileage over the life of the car, so it's worth a few bucks extra to get one.
Other benefits of the stick shift. It serves as an effective anti theft device. Most kids cannot drive stick any more, so your car is less likely to be stolen if it has a manual trannie. When climbing a snow slick hill in winter you don't have to worry about the trannie deciding to downshift at the wrong time and making the wheels loose traction.