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Discussion Starter · #1 ·
Gordon has posted that he got 42mpg in his new car at 70mph on the motorway. I know this is hardly a representative sample and the car is brand new but this is a lot worse than I expected so I have a question about fuel consumption

My wife has a mini Cooper SD (2.0 4 cyl diesel) and I understand that when driving in town she will need more power to get the DS moving due to it's extra weight and less aerodynamic shape.

However, on the motorway at 70mph she is getting 55 mpg at 2,000 rpm and I am sure that I have read that the DS will be doing 1,500 rpm for 70 mph.

My question is that if both engines are the same capacity once up to speed they will both take the same amount of fuel in each charge to the cylinder and therefore the DS should use less fuel at 70mph due to less revs.

Am I missing something?

Thanks
Steve
 

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Discussion Starter · #2 ·
As I pressed submit I had a lightbulb moment, if the turbo is running at extra pressure in the DS does that mean that more fuel would be used in each charge of the cylinder?

I think I have answered my own question :?
 

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You missed two things at cruise speed.

The air resistance. The Scx (frontale surface x CX ) of the discovery is far worse than the smaller mini cooper. In town ScX is not an issue, but weight is really a big one. in your post , you imply that aerodynamic is only important at acceleration. Air resistance is correlated with the square of the speed. At 50 miles/h, the air resistance is 4 time less than at 100.
In a lesser degree the resistance related to the tires are worse on the big weels of the disco rather than the smaller ones of the mini.
There is also the quality of motor : inner motor friction and efficiency.
 

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Discussion Starter · #4 ·
Hi Powerdoc, I understand the air resistance during acceleration but not at cruising speed. If we imagine a mini and a DS with identical engines then for both vehicles to overcome the wind resistance you would expect the DS to need to be revving higher to attain the same speed and therefore using more fuel but this bring us back to my original question with the DS actually revving lower (if I have got my fact right about 1500 rpm for 70mph)

Regards
Steve
 

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revving is just a question of gearbox in practice with such powerful engines
1500 is for the 9 th speed on the DS
at cruise speed, the mini will employ 5 % of the engine power and the DS 8 % . Only at maximal speed, you use 100 % of the engine power (and you eat a lot of gazoline)

1500 is better than 2000 for internal friction ( a quarter less exactly) but air resistant is what cost more in energy.
 

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Hi Steve
You have answered your own question. The fuel consumption at equal revs is related not simply to cylinder capacity, but rather the amount of fuel injected into the engine in each cycle. If the disco is running with a richer mixture, it will use more fuel.
 
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