Why is AT less efficient?
 

I know its well known that automatics are less efficient, but why? Is it because of the torque converter (slipping), and pump (extra work), shift points? At a constant cruising speed with the TC lockup engaged, would a AT get the same mileage as a MT with the same gearing?

You're right on all points, and "yes" to your question.

Automatics restrict how much freedom the driver has in choosing how much to load up the engine. (EG you can't short-shift an automatic.)

Also most automatics are ill-suited to being used for one of the most effective ecodriving techniques: engine-off coasting. (Most autos rely on an input shaft-driven pump for lubrication. With the engine is off, these can be damaged by excessive coasting.)

As for "why", there are several reasons:

1. Slippage
2. Heavier
3. Usually less gear selections
4. more internal losses
4a. has a larger internal oil pump
4b. takes power to shift gears
4c. takes power to decide what gear to use
4d. more moving parts and gears
4e. electrical drain to lockup the converter

and
5. like Darin says, they have a mind of their own. Not necessarily FE biased either and unable to anticipate the environment.

Also I would guess that they burn more fuel at a stoplight at idle since they are still applying torque to the rear wheels.

Good point - rarely mentioned.

I am by no means promoting automatics but, I think there are things you can do that will force the transmission to shift where/when you want it to.

If you put them in Neutral at a stoplight you will use less fuel at idle :)

That's true, Larry, within limits (IE lift to shift; but there's little you can do to achieve tc lockup beneath the set speed).

I'm REALLY glad I have a manual.:)

Slushbox
 

Yup, I miss the manual dearly. Although, on many Honda automatics, you can short-shift from 1st to 2nd, and overcome the hill-logic control by shifting down into 3rd to shift into 3rd on a grade. Then D4 is where it has a mind of its own. TC lockup wants 25-35% TPS input for full-lockup in sub-60F temps -- I don't drive more than 20% if I can help it...and 3-4 and 4-3 shifts are unpredictable.

I don't know what kind of fun stuff is going on in there with engine-off coast. I guess ignorance is bliss until it quits working. From a financial standpoint, I may have to decide on trans. longevity and/or a manual swap if/when it fails all vs. FE. It's one of the only ways to to get the Teg to achieve reasonable in-town FE. Ugh, what to do...I'm not selling selling it, so that should save some posts.

RH77