Fuel economy

[Klausy]

Hi All,

Here's one to get the grey matter going.

I have a 2.0 petrol, W203 on a '51 plate (saloon). I have to drive 55 miles each way to work and back every day on the motorway.

I am getting between 40/42mpg (yes really) due to sitting at 63-65mph with cruise control on.

Would I get better or worse mpg if I go a little faster, say 70/73mpg?

I'm thinking that there must be an optimum speed/fuel economy pay off but what speed is it that would give me the best mpg?

Does anybody want to tell me how much quicker my journey would be as well.

Cheers

 

[Dieselman]

The fuel economy would go down significantly, but you would save 5 minutes.

 

[PJH]

At what rpm is maximum torque ? Does this correspond to your cruising speed ?

 

[Klausy]

What is the maximum torque?

 

[kemsing]

Motorway MPG

I pondered this type of question myself on a motorway commute! I am now retired so cannot answer but....you have a golden opportunity to do some original research. Why not start at 60mph on Monday, fill up and work out your mpg. Then on Tuesday cruise at 65, Wednesday at 70 etc. Then you would really have some definite answers..to be shared with us of course. I suspect the curve is fairly flat over 60 - 75 mph.
Sean.

 

[Dieselman]

Since wind resistance increases with the square of the speed, it's fairly easy to see what will happen.
At 70mph the wind resistance will be 36% higher than at 60mph, so it would be fair to assume that the engine has to produce more power to overcome this additional resistance. More power means more fuel.

 

[st4]

Best MPG will be attained below 63mph!!!

 

[Richak]

I have a W204 CDi (3 litre v6) and over the last 2 weeks I have been doing a little testing on this very subject - cruising at 64mph on motorway I can get up to 42.5 on average but if you cruise at 70mph it drops to about 35.9 - weather conditions much the same

 

[Stratman]

Think about what you do with the accelerator. To go faster you press it down, allowing more fuel into the engine. It's still making the same number of bangs per mile, but each bang is consuming more fuel. Ease off and you let in less fuel and go slower.

Going faster uses more fuel, going slower uses less.

 

[artyman]

Another imponderable, is it better to use cruise for a steady speed, or a sensible right foot, where you can slightly increase speed when going downhill under less load then allowing it to drop off going uphill perhaps below your target cruising speed.

 

[Shude]

Another imponderable, is it better to use cruise for a steady speed, or a sensible right foot, where you can slightly increase speed when going downhill under less load then allowing it to drop off going uphill perhaps below your target cruising speed.

Let's not go there eh?

 

[LTD]

1 lb of pressure extra in your tyre pressure and 'hyper-miling' in the style of the Toyota Pious drivers could give you the economy you crave.

However, being a pest and sitting at 56mph might well be the best solution - it just means getting up a little earlier ...

 

[Deleted member 37751]

When I used to travel to different sites throughout Yorkshire, I would leave home at the last possible minute and arrive with seconds to spare.

I did this in a Citroen Berlingo 2.0 HDi, which was my most favourite van ever in the world because you could do 100mph on the motorway and still get a decent mpg return.

My time is more important to me than saving £20 a week on fuel, which I know can add up to a few quid, but nothing could beat that extra ten minutes in bed in a morning....

 

[Beno]

55mph will normally give the mpg

 

[Shude]

55mph will normally give the mpg

Incorrect as 50mph is easily better!

It's different for every car but I think on this forum we came to the conclusion that it's the slowest you can comfortably go in top gear.

 

[Klausy]

Surely interia must play some part of this conumdrum?

To get the car to 70 rather than 60 must use more fuel but only an imeassurable amount. When the car is at 70 would it not then take the same amount of mpg to keep it there?

I have noticed more cars now doing far less speed than say a couple of years ago. When I am travelling at 65mph I'm far from the slowest thing on the road.

 

[Shude]

When the car is at 70 would it not then take the same amount of mpg to keep it there?

Not even slightly as the air resistance goes up exponentially, a few mph turns into twice the effort.

The bugatti veryron supersport needed another 200bhp to get an extra 10mph on the top speed!

 

[t567wab]

When I've had Vito mk1 and drove it in Germany on the motorway,I've tested it.It took about 15% more diesel at 70mph than I've floored it on the way back at 85-95mph.Couldn't believe it myself.We're talking about 500 miles one way.

 

[Beno]

In general the heavier the car the lower the "Ideal" mpg speed (high road load) the lighter cars will be able to run at higher speeds (Low road load)

 

[st4]

Minimum speed without straining the car in its highest gear. 55mph for me yields 50mpg from my E320cdi