Why do stick shifts get better mileage than automatic transmission?

[JVS]

[Wickerman]

1. The guys on car talk said they don't anymore

technology has levelled the playing field. With computerized shifting the manual may soon become a thing of the past. Sorry, no link, going from memory here. AND, a leap of faith that they know what the hell they are talking about... :D

[mark414]

6. i sure hope not

i hate automatic...bah, manual til i die

[punpirate]

11. This may be true of newer technologies...

... such as the SMG transmission in the BMW M3, which is essentially an electronically-controlled manual transmission with an electrically-operated hydraulic clutch, but any transmission still dependent upon a torque converter will have some losses.

They are right in the sense that electronic controls and electronic lock-up have greatly decreased the disparity in mileage. But, whatever the EPA mileage sticker may say, if you spend a lot of time in stop-and-go traffic, you're going to see a difference in mileage between an automatic and a manual.

[billyskank]

19. They certainly do

Check the figures quoted for any new car, compare the consumption figures for manual and automatic cars with the same engines. The manual will get significantly better mileage. I observed this when getting my new car at the end of 2002.

[Vincardog]

2. Less weight and less loss in the cluch

[Salviati]

3. I also always assumed...

that since manuals typically have more gears, it was more often in the optimal gear for whatever speed you were going...

(Edited to make it say what I meant it to...)

[JVS]

5. Automatics usually have less gears

[Salviati]

9. Doh!, that's what I meant...

[MnFats]

12. actually, "fewer" rather than "less."

"less" is something that can't accurately be measured, "fewer" is something that can.


As in, "Jerry's fans were sending less healing vibes when he went into treatment for the last time."


sorry. it's the editor in me.

[madrchsod]

4. takes more energy to push fluid

but the newer transmissions shift by comupter so there is only a small difference now.

[punpirate]

7. Mostly because it's the difference...

... between having a clutch and a torque converter. The torque converter in an automatic transmission is made to have some slippage at lower speeds (although newer ones have what is known as lock-up at higher speeds which positively engages the engine to the transmission).

The clutch in a manual transmission positively engages all the time the car is moving (unless you disengage it to shift gears), so all the engine power available at a given rpm is transmitted to move the car.

Cheers.

[imenja]

10. damn, is there anything you don't know?

:wow:

I'm continually impressed.

[punpirate]

13. Plenty...

... I don't know. But, I keep learning. :)

Actually, when I couldn't find a job after college (with a couple of degrees in the humanities), I started working on cars for a living. Did that and other technical jobs for most of the last twenty-five years.

[imenja]

14. I've seen posts on medicine, history, political science, and now cars

You're widely read and very well-informed.

[punpirate]

16. There's something to be said for...

... a good liberal education. :)

Of course, when I said that to my fundie students, they all gasped, so I had to explain. I think the principles go back to Mortimer Adler (and, therefore, ultimately to Aristotle), but I tried to explain that college was meant to do two things--first, teach one how to think, and second, give one all the skills necessary to teach one's self throughout one's lifetime.

I'm just the product of a good liberal education, although there are few generalists around these days, in this age of specialization. :)

If I hadn't been, though, I probably would have starved. :) Hunger is a great motivator. :)

[ConcernedCanuk]

17. About that "slippage"

.
.
.

The way a torque converter works is basically two drums with fins, facing each other inside a sealed drum. One set of fins "throws" the oil into the second set of fins, and when they can't take the flow anymore, they start turning also, and the car moves

The second set of fins never quite catch up, although pretty close at high speeds unless the engine is working hard(accelerating, towing, steep hill, etc.): there's the "slippage"

just picture one house fan plugged in - and the other one not

turn the one fan on, and as you move the other fan closer, it starts to turn on it's own

Obviously, this fluid pushing against the other set of fins creates lots of heat - so cars where the idle is set too high, will burn the transmission out early in it's life because of the constant excessive heat

Taxis and police cars have extra coolers for the transmissions as they idle in gear alot, and that also creates alot of heat -

Can't have their trannys burnin up while they're havin their donuts now can we?

"Trailer towing" packaged vehicles with Automatics usually include an extra cooler for the tranny also - the more you haul, the hotter that tranny will get . .

And "Brake-Stands" ? -

ya know -

where you hold on the brake, and push on the gas to get the revs up so when the light turns green, you take off like a bat out of hell?

Or just to fry up a back tire or two . .

That creates one hell of alot of heat, and much of THAT will shorten the live of the tranny by a long shot

Fun while it lasts tho

If it has a lock-up converter, you save on the highway miles, but not around town.

Most lock-up converters don't kick in till over 40mph, higher if you are tramping it.

[CanuckAmok]

8. Automatics...

several reasons:

1) automatics are generally programmed to "shift" to easily, prompting uneconomical downshifting. When one drives a stick, one tends to under-rev the engine during shifts, which is better on cas consumption.

2) Automatics are still trying to make the vehicle travel forward at a stop, the application of the brakes stops the momentum, but the tranny is still attempting to travel forward.

3) The torque converter in an auto transmission is usually a slight gear reduction between the engne and the drive wheels. If you were to measure the RPMs in two identical vehicles (one with a manual gearbox in top gear, and the other an automatic) traveling on two identical surfaces at identical speeds, the manual's final drive RPM would likely be marginally lower than the automatic's. Th end result is less gas consumption per mile.

4) auto trannies are usually set up with less gears than are manual ones. Most auto trannies are three or four speeds (more and more are 5-speeds, however), while most passenger car manuals are 5-speeds or even 6-speeds. The ratio between each gearset is smaller with more gears, and therefore there's more options to choose from, as fas as optimal economy-to-performance gains.

5) auto trannies usually weigh more, and require the additional weight of their own systems of electronics and lubrication/coolling.

5a) (edit) automatic trannies are completly viscous-relaiant, to they have to carry a large amount of heavy lubricant. Also, it takes more energy to move gears through liquid than through air. Most manual gearboxes have very little lubrication compared to autos. Manuals are basically "dry-shift" gearboxes. The clutch is there to completely disengaged the geatbox from the engine, so shifts can be made cleanly from gear to gear without lubrication.

[Boo Boo]

15. In the past it was more

gears and a clutch that locked up all the way that made manual transmissions more efficient. As a poster noted above, that's not necessarily true anymore.

Automatic trannies keep getting more gears all the time. The new VW Jetta has a six speed automatic transmission. Torque converters have used lock-up designs for some time now. And, finally, computer controlled transmissions can shift to maximize economy better than the overwhelming number of drivers out there could do it for themselves.

So, not too sure the pretext of this question holds true anymore.

[politicat]

18. Sticker still shows a difference.

We went looking at Jettas today ( I want that TDI, dammit) since in CO the dealerships are closed on Sunday, and we can look without the ravening hordes of sales people descending.

The lot had an automatic jetta with a TDI and a manual jetta with a TDI; the auto sticker had 34 city and 39 highway for mileage; the manual had 37 city and 44 for highway.

I'll keep a manual for as long as possible; if nothing else, it's far easier for me to drop a manual trannie and replace it myself than for me to drop an auto.

[CanuckAmok]

20. My brain just exploded:

I just had a glimse of trying to drop the box out of a new VW product. It'd be easier diffusing unstable Soviet ICBMS in the dark.

KABLAM.

[JohnnyCougar]

21. I had to swap trannies in my Audi

shudder...luckily I know a mechanic pretty well who gave me a more than fair price! Of course, a more than fair price on that still hurt like hell!

[Boo Boo]

22. The Jetta I was referring to

is the very latest---manual transmission models haven't actually shipped yet. When the manual does ship, it will have five gears to the auto's six. We'll have to wait until then to see if the auto finally surpasses the manual for efficiency.

My Maxima has an auto because I got tired of riding the clutch during the daily stop and go commute. It's a '99, and there's no doubt that the manual version gets better mileage in controlled EPA dyno testing. The manual has an extra (overdrive) gear which improves cruising efficiency and allows for shorter gearing in the lower gears. And, while modern lock-up torque converters are a big improvement, I think they still slip more overall than a manual clutch so that might have some impact on the city cycle.

But the differences are small, and unless you know what you're doing you'll lose those few extra city miles per gallon extra by shifting late. I had a car way back when that had a little dashboard light that blinked when it was time to shift for greatest mileage. I was really surprised at how "wrong" my shift points were. That TDI, for instance: you don't have to over-rev by much to blow off those 3 extra city miles, but after 40 minutes crawling in SoCal traffic that auto is really earning its keep.

Still, if it wasn't for a really nasty commute, I'd prefer a manual as long as the car had a good one. OTOH, that Jetta six speed looks pretty damn cool!

[TreasonousBastard]

23. Most of the replies are close, but...

a lot depends on how you drive.

Modern automatics have electronic controls to optimize things and lock-up overdrives when you're cruising on the highway, so things are pretty even now.

But-- automatics still have the engine revving higher at idle and most of them have shift points more for performance than economy, so if your driving stop-and-go or in the hills the auto has a distinct disadvantage. If you've got a lead foot, it has a real disadvantage.

I drove my 5-speed very conservatively to save gas-- usually shifting up a little too soon and down a little too late. I tried to keep it in the highest gear I could, and often coasted in neutral. These things you can't really do even with the "shiftable" slushboxes. And, until you're in that locked up OD, you're still losing a something in the torque converter.

Click & Clack insist there's no difference now, but the car I'm driving has a tach for some silly reason, and even though I usually can't feel the shifts I see the engine revving up when it decides to shift at times I probably wouldn't. I get great mileage, but I can't help thinking I'd do a little better with a stick.