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-   -   Automatic Transmission = Lower Mileage : Why? (https://www.team-bhp.com/forum/technical-stuff/17584-automatic-transmission-lower-mileage-why-2.html)

are you suggesting that at a steady highway cruising speed, AT and MT have different slippage between engine and wheel?

Well you are right as far as old gen autos are concerned. Modern autos have enough speeds (Lexus has and 8 speed and Merc 7 speed. Have you seen a manual with that many?) and some have economy and sport settings that will rev the engine higher or lower.
More newer ones have fuzzy logic that try to read the driver's mood for speed and adjust the shift pionts accordingly.

Most importantly, many of them (new Civic) provide paddles so that the driver can shift when ever he wants. How can we explain low FE is this case?

Only the 2 points mentioned by Drifter and me are the biggest contributors. During transient speeds especially, the auto is notoriously inefficient due to the torque converter slip.

CVT is better than conventional auto but not as good as manual. Ask any NHC CVT owner. Otherwise Indians would have lapped up the NHC CVT in high numbers. Shift time has nothing to do with FE. Ask any drag racer and he will tell you that autos shift faster than any human.

The manufacturers themselves quote better FE figures for M/T than A/T's. M/T's giving better FE is not just opinions of 'non-A/T-users'. I think the guys have explained things quite well.

Yes, it is because of torque convertor issues as drifter explained that causes auto trannies to lose some efficiency. Generally at crusing sppeds, the difference is quite less, but in traffic with continuous differing speeds, losses are quite high.

Drive the AT/MT civic and feel the difference. In the AT, you can certainly feel that the linkage is not direct. It always feels that you are driving with a slipping clutch. OTOH, the shifts are very smooth, quick and there are no jerks.

As mentioned, comparing Honda Accord MT vs AT, the AT will have lower fuel economy. Reasons being, as described earlier:

1. The transmission losses - AT will consume some power for auto operation in terms of hydraulics etc and/or if it has a torque convertor then due to slippage which will be wasted as heat.
2. The car performs best accelaration or fuel economy depending where you are operating the engine. At the lower end (for petrol cars), it gives better fuel economy and at higher end better accelaration. Thus with manual transmission, that is with higher number of gears, you have a better choice of keeping it closer to operating point for fuel economy by changing gears earlier (same goes for accelaration from standstill where you change late).

Well firstly no one is talking about the fuzzy logic trans nor the new merc/lexus 7 and 8 speed shifts, i think the query was for the regular autos available in india coz anyone who can afford to buy the merc does'nt cre two hoots about the FE.secondly there is no setting in any car except two that is economy (regular) and sport (which changes the shifts in higher rpms) no third option is available thirdly any drag racer will also tell you that the torque and power and rpm settings in a a dragster differs to a normal everyday use car and that when the car and we're not discussing dragsters. Then again, having paddle shifts or triptronics do not change the habits of car drivers andthese are not short shift autos like in dragsters . Yes your opinion is also a factor but not the single largest contributing factor in this issue as the t/c slip is consideranly reduced in modern cars too. all these collectively add up but to say that just one point is singularly responsible...no way!!

Also have you noticed that sudden surge of torque you get when you start off from standstill in an automatic? Auto boxes are designed to give close to maximum torque on startup...so that they dont have problems in climbing up hills. But in city traffic all that torque is wasted as its not really required.

In a manual, we compensate this by applying varying amount of throttle and clutch. But in the auto, they dont have that option, its all torque or no torque so to say. Hence another reason for lower FE.

Drifter

The primary reason for lower FE in an AT has to be that an AT will operate at lower gears for significantly longer periods of time. My guesses as to why this should happen is sketched below.

MT gives more flexibility in choosing the speeds at which you shift, which may enable you to drive in lower gear for longer periods. E.g. you are climbing slightly. As per Santro manual you should shift from third to fourth at 40 kmph and fourth to fifth at 50 kmph. But since you are climbing, you could choose to alter these to 50 and 60 kmph respectively in an MT -- so that you can remain at fifth for the rest of the climb with the added momentum. But an AT may shift to fourth/fifth as soon as you hit 40/50 (or just fourth at 40 if there is no fifth gear) and then your speed will drop, resulting in an upshift etc., leading to a loss in FE. Maybe more sophisticated ATs will avoid this scenario. Similarly while going downhill, you can downshift at lower speeds, say, to fifth at 40 kmph.

You can also use engine braking more effectively in an MT. I don't know if this leads to an improvement in FE. For example, you are approaching an intersection at 60 kmph in fifth gear -- you could choose to upshift at this speed without braking and pass the intersection in third. Whereas in an AT, you might find yourself having to brake and then accelerate, resulting in a waste of fuel.

In an MT you can also upshift to third and execute quick overtakes that may be difficult in an AT. Once you are clear of the slow-moving vehicle, you can shift to fifth -- whereas in an AT, your driving style may have to be different, and you are stuck for longer periods behind slow-moving vehicles -- and hence operating for longer periods in lower gear.

I am just offering guesses here, as I don't have much experience or knowledge of ATs.

Yes it does....modern engines use almost no fuel during engine braking. That is also one point against AT.

Drifter

That is why paddle shifts of civic AT appeal. Also civic's AT does use some sort of logic control, it does seem to find an appropriate gear even when slowing down without the paddle shifts. This helps in use of engine braking and lessens brake pad wear. Of course it is not as much as MT, but it shows that there have been considerable advances in AT tech.

i corroborate this claim.....i have an OHC AT .....its feels pretty awkward to drive it in bumper to bumper traffic .

as per what i picked up from this thread ....its only the overestimation of torque that the causes AT to drop in mileage .....baakee sab not convincing.

i have driven a volvo and a mitsubishi, and i saw that the way they respond is totally diferent. the volvo even had a switch to decide "sport" or "economy" mode, which wffected the way gear shifts happened.

but still i never faced any problems as mentioend in the bumper to bumper traffic. I can drive the cars as smmothly as i want. and i can even dictate the gear upshifts with my driving style. I have not driven an OHC though.

Very true. The AT upshifts depending on pressure on pedal. By keeping pedal to the metal, I can make either Corolla AT or Civic AT shift only at redline. Be easy on the pedal and it upshifts sooner for better FE. In fact in any AT the gear shifts are usually very smooth and seamless. No problems in any situation yet with AT.