miles per gallon

Poll Poll Which is better for MPG?

  • Lower RPM

    Votes: 9 90.0%

  • Peak RPM

    Votes: 1 10.0%
  • Total voters
    10
L

Liam-O

Senior member
 Solihull
so this weekend I've done a lot of motorway driving and it got me thinking...

my MPG goes up when motorway driving, but typically my RPM is higher than when town driving. I'd say roughly 2000rpm on town driving, 3000+rpm on motorway.

So what I'm getting at it...

1) does driving closer to peak RPM increase MPG... or...

2) does driving at a lower RPM increase MPG

and then on the back of that:

should I accelerate faster to get to peak RPM sooner... OR... get to 5th gear faster to reduce RPM?


ps. sorry if this has been asked before, and sorry if its a stupid question. I have searched google but answeres were unclear
 
Neither option is correct.

There'll be a sweet spot in the rev range that gives you the best cruising speed with a good MPG.

Too low an RPM and you labour the engine and use more fuel, too high an RPM and you use more fuel without gaining much more speed.

The ideal is to get into that sweet spot in whichever gear you need for your road speed, so 50mph in 4th at 4000rpm will be worse than 50mph in 5th at 3000rpm, but 50mph in 6th at 2000rpm may be worse once again.
 
mmm-five said:
Neither option is correct.

There'll be a sweet spot in the rev range that gives you the best cruising speed with a good MPG.

Too low an RPM and you labour the engine and use more fuel, too high an RPM and you use more fuel without gaining much more speed.

The ideal is to get into that sweet spot in whichever gear you need for your road speed, so 50mph in 4th at 4000rpm will be worse than 50mph in 5th at 3000rpm, but 50mph in 6th at 2000rpm may be worse once again.
Click to expand...

so isnt this "sweet spot" the peak RPM where the engine is producing the best ratio of speed/consumption

srhutch said:
No, its the fact your driving at a consistent speed that is improving you mpg
Click to expand...

can this be true? what about driving at a constant 140mph? will i get a better MPG than varying between 30-70mph? Im not so sure...
 
constant at 140mph of course will not be good, but we live in the real world.

my most economical run in my old E46 was 4 up to the british motorshow, a round trip of 360 miles and most of it motorway doing 85mph.
 
i recently travelled to bedford for a track day from bournemouth took it easy mostly 70-75 mph on motorway but also tried to have a very light right foot managed 38.5 mpg on computer, made up for the 12 mpg on the track :driving:
 
The faster you go, the more wind resistance there is trying to stop the car. So it takes less energy to keep a car at 70mph than at 140mph. However, the difference in wind resistance between 65mph and 70mph may not outweigh the benefit of running at 3000rpm over 2500rpm (examples only, as I don't know what the 'best' RPM figures or speeds are).

The 'peak' RPM is not the most efficient RPM, it's simply the point at which the engine is producing most power, usually because it's revving twice as fast and using twice as much fuel in the process.
 
so what is the most efficient RPM?

i thought peak rpm is where the power:rpm is at its best? so any more RPM isnt giving the same proportion of power, and the same for less... am i being stupid here?
 
If you maintaining a steady speed, the lower the rpm, the more economical it will be (more revs = more explosions = more fuel used).

If you are labouring the engine (ie load on the engine is high due to steep gradient, etc) then the throttle opening may need to be higher at a lower rpm to create the necessary torque than a higher rpm.
 
Peak RPM is simply the max revs.

However, I assume you mean peak horsepower RPM, or peak torque RPM, or peak efficiency RPM.

I can offer you the first two for a Z4M, but don't know the last.

You don't need 300bhp to keep a car cruising at 70mph, 10bhp will do that. But to get it there in the same day you set out you'd probably need 50bhp or so.

I believe that in most cars, whatever RPM you are at in top gear at 55mph is about the most economical you're going to get - and a 50bhp econobox will do much better at that speed than a 500bhp muscle car, mainly because the 1L econobox requires less fuel to keep the revs at the same cruising position as the 5L muscle car.
 
A simplified rule of thumb is this: The accelerator controls how much fuel you're dumping into the engine. If you're cruising and the accelerator is barely depressed then you're barely using any fuel. Cruising at 140mph on the other hand will require close to full throttle = using lots of fuel.

It's a little more complicated than that, but it's still a pretty close relationship.
 
mmm-five said:
I believe that in most cars, whatever RPM you are at in top gear at 55mph is about the most economical you're going to get - and a 50bhp econobox will do much better at that speed than a 500bhp muscle car, mainly because the 1L econobox requires less fuel to keep the revs at the same cruising position as the 5L muscle car.
Click to expand...

Not so much - depends on the gear ratios. It used to be a rule of thumb in the past, but not anymore. The MX-5 has shorter gear ratios so it gets better mpg at a slower speed. The Z4 (with the 6th ratio) gets better mpg at a higher speed. I wouldn't be surprised if the 6 speed Z4s are designed with European motorways in mind.
 
I used to have a car with an "instantaneous" MPG readout.

Drive at 60 rather than 70, and there is large difference in MPG.

Hard acceleration uses more fuel than gentle acceleration, as Boogle says

If you want economy, accelerate slow, drive slow

If you want acceleration and speed, don't hope for economy (there truly is no such thing as a free meal)

Trust me, I used to drive a Grand Cherokee!

Steve
 
Combination of keeping momentum, low RPM and coasting in gear (uses no fuel as it essentially acts as an air pump) will see you get better fuel economy. I managed to get 42mpg out of my clio sport (getting my coupe in all of 14 hours from now, excited much :P) using those techniques over the course of a full tank. If I drove that normally I'd get 28/29mpg at best.
 
In the M on motorway cruising i found that if you keep the RPM to abotu 3000 which is roughly 75-80mph gave me the best mpg. only done it once. its so boring and it was my first drive in france and I was careful. In uk roads u dont get that- too much traffic, idiots on fast lane, breaking unncessarily.
 
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