Why the mpg difference - auto vs manual

[Fulmar]

I had a co-worker ask me the question today just chewing the fat.  Why do manuals get better gas mileage than autos.  Now before you jump on the horse.  I do know that newer cars have really closed the gap to be within 1-2 MPG or equal on some cars, especially with the CV transmissions.  But lets step back 10-15 years when it was more than likely true.

Now I've googled the question and really haven't come up with a good answer.  Is it a weight thing, with the auto transmission being heavier than a clutch system?  Is it a rpm thing where a manual driver can more accurately gauge when a shift is needed?  Combination?  Or something completely different?

Any input is good...

As an example, in my 2006 Focus (manual) I average 1-4 MPG better than my wife's 2005 Focus (auto).  Same engine/options.

[rabbidrabbit]

From what I know, all else being equal, most auto transmissions slip a bit while running.  You are slushing things around instead of direct gear connections on an engaged manual.  I think that has been improved on more recently.

[Hornet33]

Well think about when you drive a stick, When you come to an intersection and you see your going to have to stop you push the clutch and coast to the intersection and that entire time the engine is at idle power. Really pay attention to how you drive a manual and you'll notice that the engine spends allot more time at idle power than when you drive an automatic. Same thing when your shifting. Your pushing in the cluth and letting off the gas to shift gears. In an automatic the engine doesn't let off the gas it just shifts. More idle power time = better MPG

[Kaw1000]

A auto trans holds the car back...and 5 speed pretty much is free wheeling.
Less moving parts in a 5 speed...one clutch in a 5 speed...many clutches in a auto.
less friction in a 5 speed

[68Wooley]

Kaw1000 is incorrect - a traditional auto doesn't have a clutch. Google 'torque-converter' for the correct answer. Sloshing a bunch of fluid around isn't as efficient as a direct connection through a conventional clutch.

More recent auto transmissions such as the DSG box from Volkswagen have a pair of electronically controlled clutches rather than a torque converter. These allow the next ration to be pre-selected for near seamless shifts. These boxes can achieve similar or better mileage than equivalent manuals.

By the way, coasting in a manual transmission is generally frowned upon by most advanced driving schools.

[dkff49]

Google 'torque-converter' for your answer. Sloshing a bunch of fluid around isn't as efficient as a direct connection through a conventional clutch.

By the way, coasting in a manual transmission is generally frowned upon by most advanced driving schools.

I believe that the 'torque-converter' answer is proabably one of the most likely answers to this question. I will add one other thing I think is also a big contributor.

When stopped at a stop sign or traffic light in an automatic the transmission is not completely in neutral and the engine is still under considerable load which I think can use more gas. To test this sit still in a manual with it in gear and clutch presses and listen to engine now put auto in gear and hold brake. I am pretty sure that you will notice difference in amount of load that the engine is under. When in city conditions this will have a significant bearing on efficiency.

As far as the coasting. I think when he said about coasting to stop he was referring to the braking period not coasting freely, thus leaving the car in a controlled condition. I learned to drive a manual transmission many years ago and I believe they are teaching the same thing now that they did then. It is not considered a good idea to coast freely but also not a good idea to downshift either (due to unecessary wear and tear on clutch and transmission and decrease in fuel efficiency). At some point you will need to "coast" holding the brake because if you don't the vehicle will begin to "buck" and eventually stall.

[Kaw1000]

Kaw1000 is incorrect - a traditional auto doesn't have a clutch. Google 'torque-converter' for the correct answer. Sloshing a bunch of fluid around isn't as efficient as a direct connection through a conventional clutch.

More recent auto transmissions such as the DSG box from Volkswagen have a pair of electronically controlled clutches rather than a torque converter. These allow the next ration to be pre-selected for near seamless shifts. These boxes can achieve similar or better mileage than equivalent manuals.

By the way, coasting in a manual transmission is generally frowned upon by most advanced driving schools.

Kaw1000 is Correct....there are clutches in automatic transmissions..I've only been in the car bizz for 30 years.
as a matter of fact if you want to put a small wager on it I will prove to you that there are several clutches in
a automatic transmission....so how much do you want to bet??

[dkff49]

Kaw1000 is Correct....there are clutches in automatic transmissions..I've only been in the car bizz for 30 years.
as a matter of fact if you want to put a small wager on it I will prove to you that there are several clutches in
a automatic transmission....so how much do you want to bet??

not a big transmission man here but I know that automatic transimission do have clutches in them. I am not sure how many, whether it be multiple or a single but in the older transmissions they are controlled by the torque converter (I believe). The torque converter itself is not how the power is transferred from the engine to the wheels.

A torque converter is a big donut shaped container in front of the transmission (in the bell housing) fluid is forced into this torque converter by centrifugal force. The faster the torque converter is turned by the engine the more fluid goes into the converter. This in turn takes fluid out of the clutch control device "not sure of name or if there is one" in turn this engages the transmission.

Again I am not a big transmission guru but this is very simplified and as close as I can imagine the process to be.

I am sure somebody will chime in here and correct me and/or elaborate some.

edit: I spent the early days after getting my license having to teach myself how to work on cars because I never had enough money to buy a reliable car and also did not make enough money to pay someone else to work on them for me. So since this subject came up I am also hoping that someone who knows can come in here and either confirm my very simple explanation or explain what I have wrong. The above explaination was just the only way I could imagine the torque convertor would be used since automatic car and truck transmissions are not hydraulic transmissions which are very different.

 

[MiloMorai]

[Ripsnort]

Manuals do not always get better mileage. I.E. The FJ Cruiser automatic gets better mileage than the manual version.
http://www.fueleconomy.gov/feg/bymodel/2008_Toyota_FJ_Cruiser.shtml

[Steve]

MPG is typically calculated in the  last gear, at cruise. If you take the two same cars, one w/ manual, one w/ auto, it is very common for the last gear in the manual to have a closer ratio to 1v1 than the auto. Also, the auto never really locks up completely like the manual does, allowing for slippage as someone mentioned.  These are your two reasons.  Have a nice  day.   

[Maverick]

One other factor to remember is that when at a stop the auto transmission is still putting a drag on the engnie requiring a higher idle throttle setting. You are burning fuel to overcome the hydraulic resistance of the transmission even when stopped.

More power is required to move more bits including hydraulic fluid that also creates resistance for the same amount of ground covered. While auto trannsmissions do lock up at higher speeds, most of us spend most of our driving time at lower speeds where the transmission is still in slip mode.

[VonMessa]

I believe that the 'torque-converter' answer is proabably one of the most likely answers to this question. I will add one other thing I think is also a big contributor.

When stopped at a stop sign or traffic light in an automatic the transmission is not completely in neutral and the engine is still under considerable load which I think can use more gas. To test this sit still in a manual with it in gear and clutch presses and listen to engine now put auto in gear and hold brake. I am pretty sure that you will notice difference in amount of load that the engine is under. When in city conditions this will have a significant bearing on efficiency.

As far as the coasting. I think when he said about coasting to stop he was referring to the braking period not coasting freely, thus leaving the car in a controlled condition. I learned to drive a manual transmission many years ago and I believe they are teaching the same thing now that they did then. It is not considered a good idea to coast freely but also not a good idea to downshift either (due to unecessary wear and tear on clutch and transmission and decrease in fuel efficiency). At some point you will need to "coast" holding the brake because if you don't the vehicle will begin to "buck" and eventually stall.

If you take a drivers test, and do not down shift to second gear and leave it there until you are almost completely stopped (i.e. a few feet), you will be failed.  Period.    The logic behind this is that anytime the vehicle is not in a gear, or engine is not engaged to the power-train, you do not have control over the vehicle.  If you had to use power to get out of a predicament and were in neutral, you couldn't do it.

In addition, using the brakes in any vehicle to slow down is the least efficient way of slowing down (but still the fastest).  Energy is wasted as heat.

[Ack-Ack]

I've got one of those hybrid transmission where I can either set it to automatic or manual, usually drive it manual as I typically get slighter better mpg than when I've got it set to automatic.  The only time I drive on automatic is if I'm stuck in L.A. traffic.


ack-ack

[Ripsnort]

....you will be failed. 

I knew those "Epic Failure" signs were meant to hurt MY feelings!