A Theoretical Question

[3050rpm]

Let's assume you're traveling on an open stretch of road, at an appropriate 35-40 mph, and you're in fourth gear. You come across a bit of an incline ahead of you. Here's the question: is downshifting into third more fuel efficient because although it increases the revs it also moves you up the torque and horsepower curves to meet the energy demand of climbing up the incline, or is leaving the car in fourth gear more fuel efficient because you're operating at lower revs?

[RealBmw 02]

I believe leaving the engine in a higher gear would be more fuel efficient because the engine is operating at lower RPM's. Especially if you have sufficient torque and don't have to really open the throttle to make it up the hill in that higher gear.

[witeshark]

Actually I read somewhere that down shifting is more efficient because leaving it in 4th still means opening the throttle more and the engine will work harder and use more fuel less efficiently

[Dudesky]

Why don't you try it and check the mpg gauge under the tach?

[celsdogg]

i agree with dropping down a gear. when u leave it in a higher gear, it just ends up dumping more fuel in, but not increasing or maintaining speed because the amount of power being produced isnt enough to propel the the weight of the car up the hill aganist the increased pull.

its just like Dudesky said with the mpg gauge. there is a resevoir here that i always have to go up. i watch that mpg gauge a lot more since i have been driving the manual. when i start going up that hill, the mpg gauge takes a nose dive.

however, in comparing different cars, my E30 325i has no problem getting up most hills easily in whatever gear and maintaining speed. my wifes subaru tho is a different story. going up that reservoir in the same gear makes the car slow down by a little more than 5mph.

[3050rpm]

To reply #1: I agree completely!

To reply #2: I agree completely!

Anyway, to get (somewhat) serious...

To #3 and #4: Yes, I had been looking at the gauge, and although my sense was that fuel economy was worse going up an incline in a high gear, it really was rather difficult to judge that seeing the way the needle could jump around so much, plus I think there's a bit of a lag in it. Hence I was looking for more "scientific" explanations, and I think the idea of the throttle being wide open just causes more fuel to be used inefficiently when the engine isn't generating that much power.

Part of the reason for my asking was that I recall an article in Road & Track many years ago in which they reported research done by BMW. The overall conclusion was that for maximum fuel economy, one should get into as high a gear as one can as soon as one can (no winding out to 4000 rpm and stuff like that) although oddly enough with brisk acceleration but with quick shifts, even as low as 2000 rpm, to get you up to your cruising speed (obviously this is not a technique to apply when entering highway traffic). I was then wondering if the logical extension of that then applied under all driving conditions, i.e. keep the revs as low as possible without lugging for maximum economy. Granted, not maximum acceleration, but maximum economy. On a level roadway, I'd say yes, that's the rule, but efficient climbing of an incline requires more power produced through higher revs, not just by dumping more gas into the combustion chamber.

Thanks for the thoughts!

[c1apton]

If you are in 4th gear doing 40mph you're doing more theretical damage because you "lugging" the motor at lower rpms. If you did approach an incline I would expect the motor to ping or chug and load up. (All of this is dependant on rear gears - but BMWs are "long legged" for the most part for higher speed hwy cruising) Optimal cruising power and effiency is 2000 to 3000 rpm.

[3050rpm]

Interesting point, I think somewhere implicit in my question was this, what is the optimal power/efficiency tradeoff range of revs? (I imagine this has something to do with torque and horsepower curves, but that stuff is generally beyond me.) Just curious, but do you know if the 2000-3000 rpm range is something that BMWs are engineered to, or is that a general rule of thumb for internal combustion engines?

One other point concerning highway driving-- I am rather amazed that BMW 6-cylinder engines are so efficient in the range of 70-80 mph, delivering honest fuel economy figures around 30-32 mpg, on a par with if not actually better than many so-called "economy" cars.

[c1apton]

2000 - 3000 is the average "cruising range" (low end of the horsepower/torgue range) for BMWs. If you notice when you're doing hwy speeds 55-70 mph - that's your optimum milege range but also the low to low mid torgue/hp range. Rear gears have alot to do with it but mfgrs have to compromise for all around drivability.
Just for laughs take it out on the hwy & note what RPMs are at 65mph - then compare that to what your red line is - 6800rpm or so? I'd guess around the 2800RPM range is your 65mph zone and at that the motor is still napping...lol

[xsperf]

i didnt quite read all the replys, but my contribution is, whichever gear u have to push less throttle to maintain the speed is the best gear, so if you have to go half way to maintain speed in 4th, and only 1/4 in 3rd, shift down

[3050rpm]

c1apton- Yes you're right, ~2800 rpm @ 65 mph, and plenty of revs to spare. On those few occasions when I've been able to do a sustained 4000+ revs in top gear on the highway (in other words, haulin') I really get the sense that this is what the car was engineered for. Obviously other manufacturers engineer their products differently.

xsperf- Good rule of thumb (or foot).

[BMWLotAttendant]

Ummmm, all I know is that you accelerate when you downshift. When you accelerate, you use more fuel. You use more fuel at higher RPMs. Downshifting would lessen the fuel intake and increase the speed.

So my vote is to downshift and accelerate through the hill.

[drz]

Downshift. You will use less fuel to achieve the same speed and get the necessary torque. All mainly due to gearing.

[jms]

My theory of what would be most efficent would be: what would a bicyclist do to get up the hill? since you are dealing with a smaller scale the difference would be more noticable.

[3050rpm]

Very good analogy, too high a gear, with a low rate of pedaling, going up a hill on a bike is a real strain, not an efficient use of muscle power; a lower gear, although with higher "rpms," gets you up and over a lot more easily. Good point.