Automatic or stick shift

Why do automatic transmissions get worse mileage than their manual brethren? With a computer controlling the action, you would think it would optimize shift points better than a human could.


my Wag is that the automatic is less efficent in putting the power to the road (20% loss, but I can’t back up that number). so the engine has to work harder to do the same job as the stick. and that loss of efficency translates in a loss in MPG. that is also why the auto is generally slower in the 0-60 time than that of the stick counter part. (note that MB slk is faster in the 0-60 than the stick by .1 ticks, thats german engenering)

Two reasons very simply:

  1. Increased drivetrain friction through the transmission and the torque converter. Even if the torque converter is locked, the transmission still has more friction. This friction can be relatively large at higher speeds, but I think a 20% loss is a little too high - more like 5-10% for normal driving depending on the car. For a 1998 Mustang GT, the difference is supposed to be a nearly 10% power loss. I also have a Mustang magazine here where they measure the power loss of a 1971 Mustang’s automatic as being around 35-40 hp at peak, compared to the 4 speed which is around 8-10 hp. Since the engine puts out an actual 230 hp (measured on an actual dyno, not the fake “390 hp” of the manufacturuer) or so max this is a pretty significant difference.

  2. In some cases, less optimal shift points due to fewer gears, or possibly them having a higher final-drive gear than a 5 or 6 speed would. I don’t think this is a big factor anymore though.

I think with most new cars the difference between manual and automatic gas mileage is relatively small nowadays, due to a focus on increasing the efficency of automatics.

I think if you equalized it some, and made a computer-driven servo system that shifted a manual transmission you might see better gas mileage than any option, but there are a lot of issues with that. And Continuously Variable Transmissions (CVT’s) have been hailed as the cure-all since the 1970’s, as they were supposed to be an automatic with the gas mileage of a manual. But somehow they never seem to work well enough in real life to appear on too many cars.

The…call it 20% hp loss…is from the auto transmission which is fluid-coupled instead of direct-gear coupled to the driveshaft. But on most modern cars, it doesn’t make very much difference at the gas pump because the computer keeps the car in the right gear for best economy at the required accelleration. My 2000 Honda Civic auto gets the same mileage as my friend’s 2000 Honda Civic 5 speed, so I can drive around trying to dial my cell phone while he has to spend all his time driving. Fortunately enough people respond to my ‘honk if you love Jesus’ bumper sticker to keep me in my lane most of the time.

VW’s new ultra-economic 3L Lupo (3L: Uses 3 litres of diesel to drive 100 km (sorry, too lazy to do the math right now, but it’s pretty darn impressive for an every-day vehicle)) has an automatic transmission. The problem is that apparently the most economic driving style is rather uncomfortable - the transmission doesn’t shift when you expect it to and the ride gets jerky.

The car has some very interesting features for a production car: The engine cuts out after 10 seconds idle and is automatically restarted when you prerss the accelerator, the tires are a special compound etc.

But what I’m trying to say is: An automatic CAN be more economical than a stick shift.

S. Norman

OK, I’ll do it: 33.3 km/liter = 20.7 miles/liter = 78.4 miles/gallon. That’s pretty impressive - almost as good as hybrids.

Do you mean it’s still more efficienct if all other things are equal? Is there a manual version of this car to compare against? I think the OP was w.r.t the same engine and vehicle.

OK, I had to do a spot of research on that.

There’s no manual version per se, the car can, however, be driven in both automatic and TipTronic mode - i.e., the driver decides when to change gears by nudging the gear shift lever forwards or backwards, but it’s not a “true” manual, he has no clutch. Volkswagen claims, however, that their automatic is capable of more economic driving.

Seeing as the entire point of the exercise was to make a car that Joe Average Cardriver (“Walther Durchschittliche Autofahrer” ?) could drive ultra-economically, I guess it’s not really relevant when discussing the “pure” case of an expert driving the automatic/manual version of the same car. It still looks like a pretty nifty concept, though.

S. Norman