fuel consumption.. a myth busted?

[silverfoxcc]

Last year did Bracknell to Stirling M40/42/6/74/80/A80, in the Swedish brick(V70 diesel, auto) a steady 65 (indicated, sat nav true 62) and got an computed 46 mpg. Today had a bloody horrendous 90 mins from J13-26 with stop start stationary traffic etc. Where I could the old bus was on 75 indicated (72 sat nav) and I got 44 mpg. Having read the 'save the planet' propaganda I was expecting something in the low 30's. Anyone else noticed this ?

[Impasse]

Yes, my Range Rover returned 18mpg on a recent jaunt to South Wales, yet still shows 6mpg when I boot it. Barely worth going slowly.

[Depthhoar]

I raise you:

E39 530d (manual) MY2000. Currently 136k miles.

For the past month been averaging 52.3mpg one up in general open throttle motoring on NSL roads and not driving like Miss Daisy.

Heavy traffic with delays etc. on motorways gives 46.2mpg.

Once, just once, got below 40mpg and that was 4 up + luggage + roof box with skis etc. using British M-ways & 80mph+ French autoroutes to Chamonix from the Scottish Highlands.


3 litre 6 cylinder turbocharged diesel engine designed in the 1990s.......

[ferrariF50lover]

Depthhoar said:

I raise you:

E39 530d (manual) MY2000. Currently 136k miles.

For the past month been averaging 52.3mpg one up in general open throttle motoring on NSL roads and not driving like Miss Daisy.

Heavy traffic with delays etc. on motorways gives 46.2mpg.

Once, just once, got below 40mpg and that was 4 up + luggage + roof box with skis etc. using British M-ways & 80mph+ French autoroutes to Chamonix from the Scottish Highlands.


3 litre 6 cylinder turbocharged diesel engine designed in the 1990s.......

Miss Daisy didn't drive, Holk Colburn did, that was the whole point.

Simon.

Sorrynotsorry

[Alula]

I got a Jeep TJ...

If I manage to stay at 65mph I might just manage 18-19mpg.. But anything over 70mph and it drops significantly., got as low as 13mpg at one stage

Only doing avg 3000-4000 miles a year, so not lying awake about it

[aw51 121565]

"Diesel cars give more consistent fuel economy across a range of driving conditions than petrol cars do" shocker .

Most of us were brought up with petrol cars which gave high mpg under steady cruising and low mpg around town and in stop-start traffic... Diesels don't do this to anywhere near the same extent (see a couple of posts above for examples), as their design and functioning is fundamentally different to that of petrol engines .

[A900ss]

Don't forget that we are in the season where diesel cars give better fuel economy as the are on regular diesel and not winter diesel.

To make a comparison valid, you'll need to be on the same type of fuel.

[cure]

Depthhoar said:

litre 6 cylinder turbocharged diesel engine designed in the 1990s.......

They don't want you to save fuel. They want you to drive gutless cars.

[E65Ross]

I find the difference between new and old engines on the motorway isn't THAT much (but it is there), but around town there is a significant difference.

I can get a fair bit over 30mpg on a run in my 745i (4.4L V8 in a nigh on 2 tonne car), but around town it's pants, mid teens as a guess.

[Raize]

Petrol cars don't really make sense on roads with speed limits as you want the throttle wide open in a high gear at a constant speed for best fuel economy and to do this the majority of cars end up at well over 100 mph.

[E65Ross]

Raize said:

Petrol cars don't really make sense on roads with speed limits as you want the throttle wide open in a high gear at a constant speed for best fuel economy and to do this the majority of cars end up at well over 100 mph.

Err what?

[crostonian]

My 180k mile Alfa 159 1.9 diesel averages 43.5mpg and I have quite a heavy right foot, the wifes free to tax Alfa Mito Twinair does 38mpg, with a much lighter foot than mine. A farce really.

[jamieduff1981]

In my usual non-PH style, I can report vast differences in average fuel consumption in a all of my cars depending on driving style.

I asked my friend, Physics, and he agrees.

[anonymous-user]

I know what you mean. Only the other day i was Vmaxing my 335d and it only was doing 8mpg! It's all the manufacturers fault, they said it would do like 40mpg or something. And then the other week, i didn't drive the car at all for a few days, and it hardly used any fuel whilst it was just sitting on my drive...................


;-)

[Matthen]

aw51 121565 said:

"Diesel cars give more consistent fuel economy across a range of driving conditions than petrol cars do" shocker .

Most of us were brought up with petrol cars which gave high mpg under steady cruising and low mpg around town and in stop-start traffic... Diesels don't do this to anywhere near the same extent (see a couple of posts above for examples), as their design and functioning is fundamentally different to that of petrol engines .

Not convinced - 50mph - 60mph in the focus and it will return 60+ mpg, Sit at 70 mph you get about 54 mpg. Doing short journeys in town, 34 to 36 mpg isn't uncommon.

Doesn't matter what fuel the car uses, you still need more energy to go faster. (and obviously doing 0 mph in a queue is the worst possible situation to be in)

[996TT02]

Raize said:

Petrol cars don't really make sense on roads with speed limits as you want the throttle wide open in a high gear at a constant speed for best fuel economy and to do this the majority of cars end up at well over 100 mph.

One for the Fiction department.

[XJ Flyer]

996TT02 said:

Raize said:

Petrol cars don't really make sense on roads with speed limits as you want the throttle wide open in a high gear at a constant speed for best fuel economy and to do this the majority of cars end up at well over 100 mph.

One for the Fiction department.

It might work if you totally ignore the laws and basics of any fuel map which says that fuelling is directly related to load and engine speed and it's the high load high engine speed conditions which are about as bad as it gets.

[Tango13]

XJ Flyer said:

996TT02 said:

Raize said:

Petrol cars don't really make sense on roads with speed limits as you want the throttle wide open in a high gear at a constant speed for best fuel economy and to do this the majority of cars end up at well over 100 mph.

One for the Fiction department.

It might work if you totally ignore the laws and basics of any fuel map which says that fuelling is directly related to load and engine speed and it's the high load high engine speed conditions which are about as bad as it gets.

You also need to somehow bypass aerodynamic drag.

[XJ Flyer]

Tango13 said:

XJ Flyer said:

996TT02 said:

Raize said:

Petrol cars don't really make sense on roads with speed limits as you want the throttle wide open in a high gear at a constant speed for best fuel economy and to do this the majority of cars end up at well over 100 mph.

One for the Fiction department.

It might work if you totally ignore the laws and basics of any fuel map which says that fuelling is directly related to load and engine speed and it's the high load high engine speed conditions which are about as bad as it gets.

You also need to somehow bypass aerodynamic drag.

That's one of the main factors in the load requirement in the fuelling map.Basically fuel consumption is directly proportional to the throttle position the more the throttle input the more fuel will be put in with some variation in addition depending on engine speed.

[anonymous-user]

At high speed, road load is high (lots of aero drag) but relative engine friction is low (your are in a high gear)

At low speed, road load is low (no aero drag) but relative engine friction is high (low gear, so engine has to turn lots of times per unit of distance the car travels)


Somewhere in between, there is an optimum road speed for min consumption!