Driving for max MPG

Adam_151 · 24 Apr 2007 at 15:04

Its a habbit I picked up from my father to use the highest gear possible when travelling at a constant speed and change down if you need to accelerate etc, it was my understanding that this kept the RPM of the engine as low as possible and saved fuel, but just been reading some stuff on the IAM* website and came across this:

http://www.iam.org.uk/NR/rdonlyres/397E5739-15A1-4BC6-965F-5BFFEF427FAD/0/fac16001.pdf which would seem to indicate that the reverse is true and that a higher gear uses more fuel. :eek:

I assume from the statistic in the article that I can't be the only one on these boards who shifts into 4th or 5th upon reaching 30mph in the belief that this maximises MPG? If its true (and I can't beleieve IAM would be mistaken! then I better stop doing that

)

*Was looking into IAM, wondered if it might be worth doing the course to improve my driving confidence becuase I'm generally pretty nervous about driving and don't consider myself that great at at it (thats probably why I'm quite nervous)

paradigm · 24 Apr 2007 at 15:09

The less open the throttle is, the less fuel you are using.

I regularly use 6th at 40MPH, with the throttle barely open, this sees me at around 38-42MPG constant. I would NEVER think of accelerating like this though.

If I use a lower gear (2nd, 3rd), if I so much as breathe on the throttle I can see single figure MPG's.

Bobbler · 24 Apr 2007 at 15:17

paradigm said:
If I use a lower gear (2nd, 3rd), if I so much as breathe on the throttle I can see single figure MPG's.

LOL, I know that feeling now unfortunately. Never had a turbo or a car with an MPG computer in it before. Was scary how quickly it jumps from 30+ to a dolphin killing 8MPG with the blip of the throttle on the motorway in 6th.
I was always taught that the appropriate gear was the one that allowed you to make smooth progress without labouring or racing the engine. Going up hill in fourth and struggling is worse than going up it in third as far as MPG goes IIRC.

lozza23_uk · 24 Apr 2007 at 15:31

Bobbler said:
LOL, I know that feeling now unfortunately. Never had a turbo or a car with an MPG computer in it before. Was scary how quickly it jumps from 30+ to a dolphin killing 8MPG with the blip of the throttle on the motorway in 6th.
I was always taught that the appropriate gear was the one that allowed you to make smooth progress without labouring or racing the engine. Going up hill in fourth and struggling is worse than going up it in third as far as MPG goes IIRC.

yes becuase yu have to chuck loads of fuel at it to keep the revs the same.

there is a fine line. learning the car is the only part that matters.

laissez-faire · 24 Apr 2007 at 16:03

An engine is most efficient at the peak of the torque curve. If you are going to use any throttle or go up any kind of hill at all you want to be in a gear within the peak torque range. If you are literally just rolling down a hill or even cruising on flat land you want the highest gear possible. Usually though if you labour the engine the MPG will suffer.

lozza23_uk · 24 Apr 2007 at 16:05

my old golf peaked torgue at 6k rpm.

surely that isnt the most fuel efficiant place to change gear/hold the engine revs?

between 2-3k I find in my mondeo is perfectly acceptable, and I get a few hundred miles to 50quid.

usually doing really well then get stuck in traffic for an hour...or have to do loads of short runs.

just drive and enjoy it

Simon · 24 Apr 2007 at 16:15

paradigm said:
The less open the throttle is, the less fuel you are using.

Pumping losses and dynamic compression ratio are also lower though. This is the main reason little engines are better on fuel, for the same power output the throttle is more open and hence the engine is using less fuel to obtain the same power output.

Highest gear isnt always best for economy.

!bluetonic! · 24 Apr 2007 at 16:52

I manage 52-55 MPG in my 1.7 Dti Corsa.

It depends what you read.

Use your gears wisely.

Driving in the highest gear possible without labouring the engine is a fuel-efficient way of driving. A vehicle travelling at 37mph in third gear uses 25 percent more fuel than at the same speed in fifth gear
http://www.whatprice.co.uk/tips/fuel-efficiency.html

Surely it would depend on the gearing of the car as to which gear you drove in a which speeds. To save fuel a whole host of other factors are present: tyres, air-con, electrics, car weight etc. Ask yourself what car were the IAM using.

I suppose you have to be light on the fuel pedal without labouring the engine.