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Author Topic: Is There A Recognised Most Fuel Efficient Speed For A Car To Move At ?  (Read 10243 times)

Offline neilep

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Dear All,


Like my car ?



Nice eh ?...being delivered next Tuesday I..(I'm getting the dayglo Pink version with fluffy dice !...and a sticker in the back that says ' My Other Car is A Skateboard !)



I remember reading fuel economy stats where they would give you the figure of mpg at 56mph ?....I don't think they do this nowadays.....but I did wonder why they chose that speed !..was it just an average ?....or was it a generally recognised speed where a car travels at it's most efficient ?

If not...is there a recognised speed applicable to all cars or is it all bunkum ?...and that each car has it's own most fuel efficient running speed due to all the many variables.

so.....where did the 56mph figure come from ?
and is there a recognised fuel efficient speed applicable across the board to all cars ?



whajafink ?



 

another_someone

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In general, the faster you travel the more energy you lose in air resistance.

Against this, the lower gear you are travelling in, then the more the engine is working for each mile you are travelling, so you are more efficient in a higher gear.

Thus, in very broad terms, the most efficient speed is the lowest speed at which you can travel in the top gear you have for the car.

Ofcourse, this does also vary with the terrain (you can use a higher gear for a given speed when going downhill than when going uphill), and will certainly depend on the design of the car.

The other thing to bear in mind is that this only applies to speed not effected by traffic.  For all the cost in terms of fuel efficiency that you car loses to wind resistance, it is still small in comparison to the energy lost every time you hit your brakes.

Another factor, as far as traffic is concerned, is braking distance.  In traffic that is constantly varying in speed, ofcourse the further back you are from the vehicle in front, the more you can average out the speed differences of the vehicle in front, and so reduce the need for braking (at least that is so until the guy from the next lane decides to cut in front of you, and so causing you to brake).  On the other hand, ticking close in behind the car in front allows you to take advantage of the slip stream effect (beloved of racing car drivers and motorcyclists) which can dramatically reduce your air resistance.
 

lyner

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The worst thing you can do is to try to keep at a given speed.
The actual optimum speed depends on the engine design and the gearing.
To get the best mileage out a litre of petrol you freewheel down hills, you  keep a big space behind the car in front and you use the brake as little as necessary. You NEVER overtake! You have to drive like a pain in the backside for other road users!
 

Offline Karen W.

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I always thought they based it on freeway speeds...best fuel consumption for freeway speeds and not stop and start speeds in town.. etc..At least thats what they taught us in drivers training!
 

Offline turnipsock

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The worst thing you can do is to try to keep at a given speed.
The actual optimum speed depends on the engine design and the gearing.
To get the best mileage out a litre of petrol you freewheel down hills, you  keep a big space behind the car in front and you use the brake as little as necessary. You NEVER overtake! You have to drive like a pain in the backside for other road users!

you might be a bit wrong there.

When you dip the clutch and freewheel, the car then uses petrol to keep the engine running. If you take your foot of the accelerator, the engine will keep turning and it won't use any petrol due to the transmission causing a braking effect.

Staying as close as you can to the car in front is far more efficent. c.f. Nascar, the car in front uses more petrol and will have to pit first. Also c.f. cyclists, the guy in front is putting in twice as much effort as the guys sitting on his wheel. It's something to do with slipstream.

If you get into a position where you have to touch the brakes, that is when you are wasting fuel.
 

Offline lightarrow

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Dear All,


Like my car ?



Nice eh ?...being delivered next Tuesday I..(I'm getting the dayglo Pink version with fluffy dice !...and a sticker in the back that says ' My Other Car is A Skateboard !)



I remember reading fuel economy stats where they would give you the figure of mpg at 56mph ?....I don't think they do this nowadays.....but I did wonder why they chose that speed !..was it just an average ?....or was it a generally recognised speed where a car travels at it's most efficient ?

If not...is there a recognised speed applicable to all cars or is it all bunkum ?...and that each car has it's own most fuel efficient running speed due to all the many variables.

so.....where did the 56mph figure come from ?
and is there a recognised fuel efficient speed applicable across the board to all cars ?



whajafink ?



56 mph corresponds to 90 km/h. I'm not sure, I assume they chose 90 because it's a simple number near the average range of least petrol consumptions and near an average speed, outside city, in the highest gear.
« Last Edit: 20/03/2008 08:37:22 by lightarrow »
 

lyner

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Quote
Staying as close as you can to the car in front is far more efficent.
|Only true when you are doing the sort of speed which is highly inefficient, in any case. You absolutely must be at a speed low enough for drag and aerodynamics to be negligible.
Quote
If you take your foot of the accelerator, the engine will keep turning and it won't use any petrol due to the transmission causing a braking effect.
It may be the case for new fangled fuel injection but, for an engine with a carburettor, fuel is pulled through all the time and, with the throttle 'off' at high engine speed (i.e. Engine Breaking) more fuel is wasted than at tickover.
Leaving a big space was all about not having to keep braking when the guy in front does.

 

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