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post #1 of 5 (permalink) Old 11th April 2007 Thread Starter
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Fuel Efficiency

Has anyone got an answer to this little one?

I have a 2000 Laguna Alize 1.6.

I want to try and find out at what speed (motorway driving) is it the most fuel efficient.

I had assumed that maybe 56mph was the answer as it is always quoted in tests, but I'm not so sure. As I am now doing a lot more motorway driving I want to know what difference in economy I woud get by driving at a steady 70mph and say, 60mph, or the 'mystery speed'.

Any comments welcome.

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post #2 of 5 (permalink) Old 11th April 2007
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Hi there JordyLad, and welcome to the forums.

Good question - the classic 56mph figure was just a benchmark used in the government road testing, and for comparative purposes only.

The most 'efficient' speed will vary from car to car, and even for engine/transmission type, but a general rule of thumb method is;
  • look up your owner's handbook and locate the power characteristics for your specific car.
  • find the maximum torque figure, and what rpm it is developed.
  • if your engine is at that rpm 'sweet spot' in any gear - you are at the most fuel efficient mode that the car has.
The reasoning is simple, use too many revs - then the fuel is being burnt more rapidly than is necessary, change up too early, and you will be forever changing down to maintain progress (remember, every time you change gear, the engine is still using fuel). At the max torque engine speed, you shouldn't have to change down (unless you encounter an obstruction).

Hope that helps, try it out and see how you fare.

Best wishes,


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post #3 of 5 (permalink) Old 11th April 2007
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I would say the slowest speed you can do comfortably in top gear will be most efficient.

The faster you go the higher the wind resistance which means more power needed and therefore more fuel.

Lets say a car has 120BHP and can do 120mph top speed, for that car to do 240MPH the car will need 600BHP, speed has only doubled but power required has more than quadroopled.

So roughly speaking you need 4x more power to do 100mph than 50mph (roughly speaking)

So say the difference in MPG between 50 and 60 is 50=50mpg and 60=48mpg, 70 will probably be 44MPG.

Sorry to rant but probably be easier to explain with a graph.

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post #4 of 5 (permalink) Old 11th April 2007
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Hi I have been driving for quite a few years now got licence as soon as i was old enough and now 67 years old i have always found that to keep a car around 3,000 rpm seems to give the best mpg no matter what car i have been driving any one else find any simular experance

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post #5 of 5 (permalink) Old 11th April 2007
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Originally Posted by tomandjo View Post
I would say the slowest speed you can do comfortably in top gear will be most efficient.
I'm inclined to agree with that, though I've heard some people claim that being in a lower gear doesn't harm your fuel economy. Surely you're wasting energy purely by making the pistons go up and down more often than they have to. Or maybe that's just my poor grasp on physics.

I've found noticeable differences between cruising on the motorway at 80, at 73 (which is 70 according to GPS) and again at 70. Each time, there's a marked improvement. I once tried resetting the trip counter in a set of 40mph roadworks and let it lug along in 6th gear at about 42mph on the cruise display (40 on the nail according to GPS.) That's about as slow as you can go in 6th without it complaining and I was getting 80+MPG on the level!

Typically, the car achieves high 40's at my usual motorway cruising speed.

So yes, I'd stick with 'The slower the better as long as the car is in top gear and the engine doesn't feel laboured.'

There's also fuel to be saved in the old trucker's trick of 'draughting' (riding the slipstream of the truck in front.) I've used this trick a few times when I've been low on fuel and too lazy to fill up, and I've managed to get very high 40's out of my old 2.0 IDE that used to struggle to break 30MPG. But draughting is risky, and to get the full effect, you have to seriously compromise your stopping distance. As I said, I've only ever used this as a 'get me home on fumes' measure, and I'm not particularly proud about doing that I'm not recommending that anyone does this habitually to save pennies.
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