Greetings,

I have a general question about fuel consumption vs gearing in passenger cars. My current vehicle has a 5 speed automatic (I know, I know, that's a wife thing not a BBT thing trust me) which has a semi-manual mode whereby you bump the shifter forward or backward for shifts. Obviously, that's the way I drive since it's the lesser of two evils.

The few times I've left it in full auto mode, I couldn't help but notice that the shifting is (obviously) very lazy, and seems to be programmed to maintain the lowest RPM possible, even when moderate throttle is applied. When I'm driving, I tend to keep the RPMs in a reasonable operating range, unless I won't be slowing / accelerating for considerable time because the response is so much better. The engine has variable valve timing, but it's still only 2.3L and really doesn't have much down low. Thus, in city driving I often find myself at least one gear lower than the auto would choose on it's own.

Now, I haven't done any scientific testing but anecdotally speaking it SEEMS like I get better gas mileage this way. What I'm not sure I understand properly is the relationship(s) between throttle position and engine RPM vs consumption.

If left to it's own devices, the transmission will often require more throttle for a longer time to induce reasonable acceleration, because it's decide that some bullshit gear ratio is desireable, whereas a lower gear would provide quicker acceleration with LESS throttle application.

Will an engine burn more fuel at lower RPM but much wider throttle, for say 6 seconds, or at higher RPM but less throttle for 33% less time.

I know that's not a precise question, and that there's a lot of variables involved that need to be considered, but I hope it conveys the gist of what I'm getting at.

I have wondered this same thing myself, I look forward to the answers.

Quote from Ball Bearing Turbo :

which has a semi-manual mode whereby you bump the shifter forward or backward for shifts.

ill bet any amount of money that its the wrong way round

Quote :

Will an engine burn more fuel at lower RPM but much wider throttle, for say 6 seconds, or at higher RPM but less throttle for 33% less time.

I know that's not a precise question, and that there's a lot of variables involved that need to be considered, but I hope it conveys the gist of what I'm getting at.

as youve already worked out yourself thats pretty much an inanswerable question since there are way too many variables especially since lower rpm and higher rpm are essentially meaningless without any further qualification

however when thinking about fuel efficiency during acceleration youre essentially looking for consumption/Nm
generally and this applies to all engines the point at which an engine is most efficient is at its torque peak (which afaik doesnt move all that much over different throttle openings)
which is obvious once you think about it in terms of why the engine generates the highest torque at that point (an engine is basically a highly inefficient device that converts pressure (essentially undirected force) into linear motion (directional force) and then into rotation (rotational force ie torque))
so to conclude the most fuel efficient way to get to a certain speed (disregarding the distance traveled) is to stay near the torque peak... and to not ever under any circumstances buy an automatic

Quote from Shotglass :

ill bet any amount of money that its the wrong way round


youve already worked out yourself that this is a pretty much an inanswerable question since there are way too many variables especially since lower rpm and higher rpm are essentially meaningless without any further qualification

^^^^ this is my basic answer.

include strict perimeters and we can answer it better OP.

It's very likely that an automatic which has a manual override, when used, will give better MPG. Simply because in semi auto, it holds in gear and pulls like a manual, as opposed to the old rubber banded auto.

And as a rule of thumb the best economy is about peak torque in top gear, but of course vehicle aerodynamics and tire sizes etc can all affect that...an engine on it's own would be most efficient at peak torque...but hey...50mph in top gear, job done.

Use the appropriate gear to keep it as close to peak torque as you can as you accelerate, then a soon as you hit the speed you want, put it into top gear.

For driving at 30-40mph round town, I'll usually go 1st, 2nd, 3rd up to 40mph, then straight to 5th (I realise your semi-auto can't do this).

I've always thought about this myself. I tried my own little experiment but it didn't seem to help doing it manually (I could have been doing it wrong if thats possible lol).

All I did for 2 weeks straight was drive to uni and back each day. After each week I used 250 miles (25 miles each way). I didn't calculate the weekend driving into it because that's fun time.

Driving the same route each day, I used 25mpg using semi-auto, and used 26mpg using full-auto. Who knows, maybe I was a bit of a heavy foot.

Quote from Crashgate3 :

Use the appropriate gear to keep it as close to peak torque as you can as you accelerate, then a soon as you hit the speed you want, put it into top gear.

For driving at 30-40mph round town, I'll usually go 1st, 2nd, 3rd up to 40mph, then straight to 5th (I realise your semi-auto can't do this).

I go 2nd -> 5th

OHHHH YEAH SAVIN' DOLPHINS.

Quote from Shotglass :

ill bet any amount of money that its the wrong way round

Forward press is downshift, backwards pull is upshift. What you think about that?

Quote :

as youve already worked out yourself thats pretty much an inanswerable question since there are way too many variables especially since lower rpm and higher rpm are essentially meaningless without any further qualification

Understandable, this is why I was looking for a generalization. But it seems pretty obvious from your statements below that the most obvious assumption in this case probably is correct. Having a torque converter throws a bit of a wrench into things though. I think however that the situation should be considered with a constant throttle opening. So even if I had torque output figures for said throttle %, and the gear ratios involved, the torque converter would be a hard variable to consider (variable torque multiplication effect).

Let's say for example I went to accelerate from 30km/h to 80km/h. Let's assume that I apply 25% throttle to get there. On it's own, the Auto will have been probably in 3rd to begin with, and will shift into 4th, and subsequently a locked up 5th by the time I hit 65km/h.

If I apply the same 25% throttle but begin in 2nd gear, shift into 3rd at maybe 50km/h and remain there until I hit 80, and THEN shift directly to 4th then 5th, I would accelerate noticeably faster. Note, all figures pulled directly out of my ass.

The more I'm thinking about this as I type it though the more I realize that I'm over-simplifying it rather significantly. I was always under the impression that autos were designed with economy in mind, and the lesser mileage with autos was mostly due to pumping losses & rotating mass, but maybe they're mostly programmed to just be unintrusive for the general public dumbass and thus even more inefficient than is necessary?

Quote :

and to not ever under any circumstances buy an automatic

You're preaching to the choir. However we share a vehicle (really don't need two) so I was required to be accomodating at least a little bit

I remember reading an article in a brazilian website that did test what method were more efficient fuel consumption wise (high gear + throttle wide open or low gear + throttle more closed).
The result was that the high gear method consistently provided better mileage (if speeds aren't important). I believe all cars tested were manuals, as we don't get many autos here and it was made a while ago.

I'm at work right now but I'll dig the article when I get home. You'll have to live with Google translation though...

Lower/Higher RPM aren't meaningless because they directly affect one known, the internal mechanical resistance. Higher RPM means overcoming the internal resistance more often in the same time span, it also generally means higher oil pressure and air movement = pressure, despite ventilation (crankcase / valvetrain).

The only way to know for sure is to try it out. The ECU knows exactly how much fuel it's injecting (or at least, how much it's telling the injectors to inject) so it's the only reliable source of information. A torque curve is not, because that's WOT across the rev range and WOT is not likely to be the most fuel efficient throttle input. Might be worth mentioning that EGR is bypassed under WOT, but whether that affects fuel economy positively or negatively is up for debate (and, ideally, scientific observation).

Find the RPM and throttle position where the least fuel is injected and you have your answer, for the exact conditions you measured it in anyway.

Measuring it on carburettors seems complicated so I'm not even gonna think about it any further

Pretty much as what has been mentioned, acceleration is mostly what uses fuel. Accelerate swiftly with moderate throttle, then cruise at the lowest possible RPM with very little throttle.

Closed throttle positions are very inefficient for accelerating due to turbulence. Not much of an issue at cruise.

Quote from de Souza :

I'm at work right now but I'll dig the article when I get home. You'll have to live with Google translation though...

Here it is:

The brief technical explanation;
A bit more;
A little bit more;
Why it doesn't work with diesel or two-stroke engines;
The actual fuel consumption test.

Hopefully this is accurate. If the translation is indecipherable just ask and I'll try to make up for it. I've seen there are some weird things, like when you read 'speed' in 'Low speed and high load', consider it 'engine speed', as in RPM.

Quote from S14 DRIFT :

It's very likely that an automatic which has a manual override, when used, will give better MPG. Simply because in semi auto, it holds in gear and pulls like a manual, as opposed to the old rubber banded auto.

Nonsense! Conventional automatics (ie. gearboxes with a set number of ratios) always 'hold' in a gear, they have to, there are only a set number of ratios and the only difference between manual override and automatic modes is the point at which the ratio changes are actuated.

Conventional automatic gearboxes are not and have never been rubber belt driven! You're getting confused with CVT transmissions, only a small number of which have used rubber drive belts (the DAF/Volvo 340 being the only car example of note).

A CVT transmission is completely different due to having no fixed gear ratios, they are potentially the most efficient form of transmission as they can hold an engine at its optimum speed. They have failed to take off partly due to early designs using simple manifold vacuums to rather crudely control the gear ratio tending to run an engine above the optimum speed at low engine loads, where car engines spend most of there time operating, giving poor fuel economy. With modern electronic control CVT gearboxes could be very efficient but the characteristics of a car with engine speed independent to road speed is unpopular, many modern CVT gearboxes have modes that simulate gear changes which completely defeats the point!

I'm currently running about in a Volvo 340 variomatic, it's surprisingly nippy at low/medium speeds and certainly seems to out accelerate a manual 1.4 340 (which I also own, don't ask!) to around 40mph and just about anything off the line, incredibly amusing to see the confusion on the faces of chavved up Corsa/Saxo drivers when the grannie-mobile blasts past as they fumble about finding second gear!

Conventional autos are less efficient than manuals (regardless of what shift points are used) mainly because the torque converter is an inefficient drive coupling. Modern autos are starting to use mechanical engagement systems once at speed to reduce their drivetrain losses but still have greater rotating mass/fluid drag from having the torque converter there. On a side note the Volvo uses a completely conventional dry clutch plate engaged by centrifugal force.

I'm not getting confused with CVT. I know the difference. I was simply using that as an expression of how the gearbox and revs operate. Easier for most people to understand the sensation that way.

You can see what I mean when you floor an auto car from a standstill and it sort of hovers around 2-3k RPM until you're going at a speed which if you were in 1st you'd be going and then revs up higher and even so it's very mushy and slushy..

Unfortunately CVT gearboxes are also pointless, annoying, gay, and ruin performance and economy. (look at MPG and peformance figures of the CVT gearboxed cars compared to their manual equivalent...Focus C-Max for a start!). A regular auto is better than a CVT. Manual is better than both.**in my opinon. Oh snap.

P.s my Mum had a Vovlo 340 1.7 Redline and was a great car! Can't remember what happened to it...just remember the rubber spoiler getten bitten off by a Monkey at the drive through bit of that safari park.

http://www.youtube.com/watch?v ... aeffo&feature=related

ps icky.

Quote from S14 DRIFT :

A regular auto is better than a CVT. Manual is better than both.

It's not, it can't be, the fundamental physical principles behind CVT make it better. CVT has a bit more complex construction than an ordinary gearbox and it would probably take some getting used to and tweaking if you wanted to use it in a race car, but as far as fuel economy and acceleration goes, CVT is the way to go...

See my edit

Quote from ajp71 :

Modern autos are starting to use mechanical engagement systems once at speed

Depends how modern you mean... but autos have had locking torque converters for ages; Mazda's newest aims to be locked 80% of time in city driving as opposed to about 50% for most current gen autos.

Quote from MadCatX :

It's not, it can't be, the fundamental physical principles behind CVT make it better. CVT has a bit more complex construction than an ordinary gearbox and it would probably take some getting used to and tweaking if you wanted to use it in a race car, but as far as fuel economy and acceleration goes, CVT is the way to go...

As far as I am aware, torque throughput capacity has been (is?) their only real downside.

Eeek, I'm scared, In a guy conversation.

When I was at pep boys once, I had asked the same question as you did. (2011 Mustang .. Thank you credit.)

He had said (Qualified mechanic, Could be a idiot, idk.) Said that the lower the RPM, The more fuel you are saving, Due to the lower revolutions in the pistons, Thus less gas in the pistons is moved, Causing the engine to sit in a "Relaxed" state, Only using more fuel when you accelerate (How the hell does cruise control work then!)

It even seems logical.. To me. (The lower species.) That if you're using a lower RPM, You're not going to use as much gas as if you were going all the way in your gears.

However, I can't talk, I have killed my fair share of glaciers (And rubber.. And ozone...) driving my car. (old abandoned road.)

P.s Dye your hair blonde and drive next to guys in a hot pink/black mustang, instantly asked for your number.

Why were the guys driving a pink and black mustang?

Wrong: not "lower species"; I believe the word is "gender".

PS I could use a sammich

So glad my wife doesn't read these forums

For both of those comments you'd be sleeping on the couch tonight!

Quote from PMD9409 :

Why were the guys driving a pink and black mustang?

I was. you fool.

Quote from Ball Bearing Turbo :

Wrong: not "lower species"; I believe the word is "gender".

PS I could use a sammich

So glad my wife doesn't read these forums

Well.. I am on the bottom.. so.......

And no sandwiches for car parts. And why? Your wife would make you wear a skirt? x3

No, but thanks for the suggestion...

Tongue in cheek of course, I really should be mature enough to not laugh at juvenile remarks. Though should that day should come; I've probably died

Quote from S14 DRIFT :

I go 2nd -> 5th

OHHHH YEAH SAVIN' DOLPHINS.

You shouldn't do that. Maybe it will not ruin your transmission, but it's not good for it. I know on the S2000 it's a big "NO NO!"... it's well known skipping gears will destroy the synchros.