V70: General Reasonable fuel consumption?

[apersson850]

Sure, I can't prevent you from insisting on being wrong. That's your choice.

However, it's still a fact that the car can give you a precise value for the fuel consumption, no matter how you drive, or if you drive differently from one day to another (for example by towing a caravan one day but not the other).
The only uncertainity is in the measurement principle, since deviations in fuel density may influence it. It normally is a bit more precise for petrol cars than diesel ones.
Another reason for underestimating the real consumption we see quite a bit of here in Sweden in the winter, and that's running the engine heater. The diesel it consumes isn't measured by the trip computer, is it's not injected into the engine.

But in general the trip computer is very close to the real consumption.

[simboc2004]

And, of course, winter diesel is different from summer diesel in that it has additives to stop it waxing below freezing, which, as a side effect, increase the fuel consumption slightly.

[simboc2004]

And fuel bought on a 40C day will be greater in volume than fuel bought on a 10C day... I'd be interested to know the proportion of volumetric change in diesel and petrol as ambient temperatures rise. Petrol shouldn't;t change as much, I would have thought, because it is more volatile - a bit like Vodka not freezing in a freezer, but wine will...

I try to never fill up my heating oil tank at home in hot weather, because you must be getting a reduced amount of fuel in an embedded energy sense - I check my tank levels every month and I noticed that, this summer, (only using the oil for heating the water) the tank levels went up during July and August, from which one could deduce (completely wrongly( that the boiler was creating fuel and putting it back in the tank. Checking this month the level has dropped sharply...

Basically, it's almost impossible to achieve a set of wholly accurate fuel consumption figures. Someone else note a there-and-back motorway journey at roughly similar speeds, but with quite different consumption readings caused by traffic/gradient/weather/tyre pressures/temperature differences.

[SnineT]

Originally Posted by apersson850

But in general the trip computer is very close to the real consumption.

I'm not disputing that,I'm trying to tell you what 40 odd years of in and out of the motor trade taught me about the readout and anyone relying on it needs to know it's not accurate enough to trust.

Have you never experienced the range go from 120 miles to 60 in the space of about a mile simply because you left free moving traffic to crawler traffic? sometimes it's an overnight thing, you'll park with a range of 90 miles and by the end of the road it's dropped to 40 but it's still showing your glorious 55 mpg on the cluster.

[Pidgeonpost]

Originally Posted by SnineT

I'm not disputing that,I'm trying to tell you what 40 odd years of in and out of the motor trade taught me about the readout and anyone relying on it needs to know it's not accurate enough to trust.

Have you never experienced the range go from 120 miles to 60 in the space of about a mile simply because you left free moving traffic to crawler traffic? sometimes it's an overnight thing, you'll park with a range of 90 miles and by the end of the road it's dropped to 40 but it's still showing your glorious 55 mpg on the cluster.

Isn't the range figure influenced also by the style of driving? I've had many instances where the range has increased as I tootle along. When I returned from hols a couple of weeks ago my mpg was showing 45mpg. After a couple of weeks it's slowly dropping back to what is the norm for me. But it's not worth getting too anal about. Just the occasional check does it for me.

[apersson850]

Originally Posted by SnineT

I'm not disputing that,I'm trying to tell you what 40 odd years of in and out of the motor trade taught me about the readout and anyone relying on it needs to know it's not accurate enough to trust.

You're rather saying that if you don't understand the value it calculates, then it will not meet your flawed expectation.

Have you never experienced the range go from 120 miles to 60 in the space of about a mile simply because you left free moving traffic to crawler traffic?

No, I haven't. If you now by range are talking about the estimated range on remaining fuel, then that does of course change if you change your way of driving. If you cared to read what I wrote before, that's the effect you get if you hook up a caravan. Suddenly the consumption doubles, and of course the predicted range on the current fuel level will change to half. For the cars in this part of the forum that calculation is based on the last 30 km driven, so it will take you that long to adjust the prediction completely.

Sometimes it's an overnight thing, you'll park with a range of 90 miles and by the end of the road it's dropped to 40 but it's still showing your glorious 55 mpg on the cluster.

The average consumption, even if you display it as range (mpg) and the predicted range on remaining fuel are two different things. If you manage to get a readout of 90 mpg (downhill, maybe?) and then drive at 40 mpg, it's of course inevitable that somewhere your average will pass 55 mpg. Just like it passed 80, 70, 60 and so on.

This doesn't say it's not accurate right now. It's like saying that if you drink three pints of beer today that is inaccurate, just because you only drink one tomorrow. At the end of tomorrow your average is two pints, albeit you didn't drink that in any of the two days.

[SnineT]

Originally Posted by apersson850

You're rather saying that if you don't understand the value it calculates, then it will not meet your flawed expectation.No, I haven't. If you now by range are talking about the estimated range on remaining fuel, then that does of course change if you change your way of driving. If you cared to read what I wrote before, that's the effect you get if you hook up a caravan. Suddenly the consumption doubles, and of course the predicted range on the current fuel level will change to half. For the cars in this part of the forum that calculation is based on the last 30 km driven, so it will take you that long to adjust the prediction completely.
The average consumption, even if you display it as range (mpg) and the predicted range on remaining fuel are two different things. If you manage to get a readout of 90 mpg (downhill, maybe?) and then drive at 40 mpg, it's of course inevitable that somewhere your average will pass 55 mpg. Just like it passed 80, 70, 60 and so on.

This doesn't say it's not accurate right now. It's like saying that if you drink three pints of beer today that is inaccurate, just because you only drink one tomorrow. At the end of tomorrow your average is two pints, albeit you didn't drink that in any of the two days.

Too tired to care mate, lots going on,you keep to how you believe it works it's not important anyway, all the best.

[apersson850]

Well, it may be more important to other people than to you. That's why I've kept on trying to explain how it really works, so others don't get confused.

Have a good day you too!

[pjm84]

Well it seems to vary, so not consistant, and this time pretty accurate

599miles on 61.97 litres. Less urban running around and longer spins. Dash recorded 43.6mpg

[SnineT]

One thing for certain everybody's mpg will be dropping soon as the heated seats, demisters and lights get called upon.

Winter draws on.