Fuel Efficient Driving

Orion21

Orion21

Orion21

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20 Feb 2006
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343
I was wondering about both the most fuel efficient way to accelerate. Is it better to accelerate slowly and keep rpms low or to accelerate quickly with higher rpms?

I thought it was slow acceleration but after looking about it seems that accelerating quicker with more pressure on the accelerator pedal might be better between roughly 2k-4k rpm for most cars before changing to a higher gear.

Also when maintaining a constant speed is it best to keep rpms as low as possible (i.e. say 1k to 1.5k rpms) or is it better to keep them between a certain rev range which is higher for fuel efficiency?
 
Lowest revs and lowest throttle possible without loading the engine too much.

High revs just turn fuel into heat
 
Simon said:
Lowest revs and lowest throttle possible without loading the engine too much.

High revs just turn fuel into heat
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+1

Short shift, coast in gear as early as possible to reduce braking, change down once not many times.

Although in many cases the official figures don't back it up, modern Auto's are much easier to drive economically than manuals, especially in manual mode.
 
After a full tank of painstaking eco driving round town I got a whopping 2.5mpg more than from ragging it around. Hardly seems worth the effort!
 
I had a 120mile return trip last weekend. Ragged it on the first leg, drove in an 'eco-friendly' way on the way back and my mpg (according to onboard computer) differed by 14mpg - I'd say that scaled up its definitely worth the effort :p

Orion, be wary of the gear you are in too. For example, accelerating from 40 in 5th will most likely (depending on ratios) give you worse fuel economy than dropping a gear and accelerating due to engine having to work harder - even at lower revs. So essentially, what Simon said :)
 
I did read a study done a few years before but that was essentially use 80% of the engine torque to accelerate to your cruising speed and try to maintain that. Not everyone is going to exactly how throttle/RPM is required for 80 torque so I just try to use as few RPMs as possible and rely on torque without lugging the engine (as above). Generally changing at say 2500-3000 RPM in a petrol and 2000-2500 RPM in a diesel.

On my Audi A4 TDi that went from 42 mpg to 47 rpm average doing the same commute. Upping tyre pressures 5 PSi probably helped just as much though.
 
Drive like some of the taxi's I've been in. They are the masters of short-shifting... one guy pulled out from a junction to join an A road and was in third gear by the time we had crossed over into the left hand lane :eek:

I've tried driving to save fuel over the course of a week and the difference was around a 1.5mpg higher average... this was mostly short to medium daily runs.
 
I find I tend to drive the most efficiently when the amber low warning fuel light comes on, for some reason even though I know there should still be around 6 litres left I tend to accelerate more slowly, read the road conditions more so to minimise braking and although I know I shouldn't I also let the car coast on downhill sections.

For me the difference is only around 3mpg difference but then I suppose every little adds up.
 
Skillmister said:
After a full tank of painstaking eco driving round town I got a whopping 2.5mpg more than from ragging it around. Hardly seems worth the effort!
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How many gallons in your tank? If you can go 2.5m further for every gallon, that could add up to a noticeably longer distance covered before filling up again.

I try to lift and coast as much as possible before braking.
 
IAmATeaf said:
I find I tend to drive the most efficiently when the amber low warning fuel light comes on, for some reason even though I know there should still be around 6 litres left I tend to accelerate more slowly, read the road conditions more so to minimise braking and although I know I shouldn't I also let the car coast on downhill sections.

For me the difference is only around 3mpg difference but then I suppose every little adds up.
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If by "let the car coast" you mean driving in neutral then you're using more fuel than simply leaving it in gear and taking your foot off the accelerator. You use fuel to idle, you don't use any fuel when decelerating in gear.

It is also dangerous.
 
roadie said:
How many gallons in your tank? If you can go 2.5m further for every gallon, that could add up to a noticeably longer distance covered before filling up again.

I try to lift and coast as much as possible before braking.
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40l so what, 9 gallons? An extra 22 miles per tank sounds better but my car doesn't feel like it enjoys being driven like that. Tends to hesitate often when accelerating at low revs.
 
DRZ said:
If by "let the car coast" you mean driving in neutral then you're using more fuel than simply leaving it in gear and taking your foot off the accelerator. You use fuel to idle, you don't use any fuel when decelerating in gear.

It is also dangerous.
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Spot on. :)

As for me I short shift before the car 'gets onto the cams' - fortunately having a torquey V6 I can get away with reasonably high gears without labouring the engine.

That said, I didn't buy my car for fuel economy :p
 
DRZ said:
If by "let the car coast" you mean driving in neutral then you're using more fuel than simply leaving it in gear and taking your foot off the accelerator. You use fuel to idle, you don't use any fuel when decelerating in gear.

It is also dangerous.
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WRONG. The distance you have travelled is far further by coasting than you would have achieved otherwise.

I am a "SafEd instructor" (Safe and Fuel Efficient Driving). All new trucks for example automatically switch into neutral to Eco cruise whenever the sensors detect declines.

They also do this just before you crest a hill and use kinetic energy to carry you over the top. This of course means our road speed plunges dramatically.
 
If your car is a relatively new and has auto trans then the chances are it's auto switching into neutral without you realising.
 
I think its also dependent upon the ECU.

I know that when going down hill in gear on my car, the MPG reading on the trip goes to dash, it has no reading presumably because its cut off fuel completely to the engine so it can't calculate miles per gallon using no fuel.

Switch it to neutral however, and you can get a reading, albeit a stupidly high one like 99mpg because it can't read any higher.

"All new trucks for example automatically switch into neutral to Eco cruise whenever the sensors detect declines."
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Disagree here. Switching to neutral does nothing. Its the shutting off the fuel bit that's important.
 
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Outcast said:
If your car is a relatively new and has auto trans then the chances are it's auto switching into neutral without you realising.
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It isn't.

This functionality was introduced as part of the facelift, my car is a pre facelift. My Dad's car does do it but the circumstances in which it does so are quite limited.
 
Smooth and gradual with the throttle when accelerating, change gear smoothly and at as low of an RPM as you can without laboring the engine, keep an eye on the road ahead and compensate for distance by easing off of the throttle or decelerating in-gear with no throttle, instead of using the brakes.

Don't drive too fast on a run, there will be a sweet spot for fuel economy/speed on a run. In my car for example, it is 60MPH. On an incline which will soon peak and become a descent, don't be afraid to let the car decelerate a little rather than laboring the engine more, you will be able to use the descent to re-gain speed without using extra fuel.

In situations where you are ascending, it would be better to use a lower gear which allows you to stay in a usable power band, than it would be to use a higher gear which sits at lower RPM's, but requires more throttle input and labours the engine.

On a steep descent, you may be able to use no fuel at all by keeping it in gear and using no throttle input. On some long but gradual declines, you might be able to achieve the same by putting it in a high gear and giving no throttle input. For example on my way home from work, there are at least two stretches of road of about a mile long, where I can sit at 70MPH with no throttle input, and the engine resistance preventing me from exceeding this speed.

Tracking and alignment will help fuel economy, as will using the correct tyre pressures.

The type of tyres you use will also have an impact.

In certain cars, the quality of the fuel you are burning can also have an impact on fuel economy.

Remember that auxiliary functions like climate control also sap a small amount of engine power/potential, so in extreme hypermiling circumstances, you could even turn it, and other things, off.

Think that covers everything really... :)

I usually get about 34MPG on my commute, but using all of the above tactics, I can get up to 41MPG.
 
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