V70: General Reasonable fuel consumption?

[SnineT]

Originally Posted by yostumpy

Had my D5 18 months now, always check mpg with a fill up to a click, then top up to nearest .5 lt. Always within 1.0 -1.5 mpg less than the readout, whether it’s a 400 mile 60 mph motorway jaunt , or a 400 mile 38 mpg …ah! this bloody traffic, type jaunt. Some things you just know.
In another life, I used to do antique fairs, and a tally at the end of the day , would always, no matter what or how much was sold, mean cost of stock was around 25% of the takings. As I say, some things you just know.

Thx Stumps, if you look at Yo's fuelly it shows an average of 42 mpg which is about right for a heavy estate car, there will be times it was returning over 50 and times it couldn't do 30 but overall it said an average of 42 which is what I was trying to say, however if it was used urban all the time the figure wouldn't look too clever and might even be as low as 25 mpg, the engines I was talking about were the 5 cyl 2.4 units which however good they might have been weren't able to return what the 4 cyl VEA can.

Also once warmed up a diesel comes into it's own along with once it's in top gear it's in sip mode hence why I said about my Insignia being lousy in town but really good on a run and even the larger diesel engines benefit this way as power to weight comes into play.

[apersson850]

Originally Posted by SnineT

Sorry to have to tell you this but the average isn't worked out across the tank by the car, it's worked out over a mileage period that varies normally over the previous 40 miles, the S90 you can auto reset and it reports increments of your overall consumption.

This mean that if you did 50 miles gently followed by 30 miles of crawling and stop starting the dash might say 45 mpg but the reality is when you fill up it may work out as low as 35 mpg because it's mixed the averages up for different scenario's you've driven.

I'm not sorry, but you should be, since you are completely, utterly wrong.

The average fuel consumption is calculated by taking the sum of all fuel consumed since last reset divided by the distance driven since last reset. That's it. Nothing more to it.
For the dashboard's trip computer, the reset is always manual. For cars with trip statistics in the center display, you can select to either reset manually or to reset automatically for each new trip. A new trip is defined as starting at least four hours after the car was running last time.

The average speed is computed in the same way. Divide the distance driven since last reset by the time spent driving it. Manual reset there too.

The only thing that does work as you describe is the range on remaining fuel calculation. That's based on the rolling average consumption the last 30 km. Thus if you drive on the highway for at least 30 km, then you have a prediction that's fairly accurate of how long you can continue doing that before you have to refuel. If you stop to hitch a caravan, it will take another 30 km of driving before that caravan's full impact on your range is reported correctly.

What kind of dreams people get about fuel consumption is another thing. If you encounter a car that's been rolling on highways most of the time, and where the trip computer hasn't been reset for ages, and you start doing mainly urban driving with it, then your consumption will be significantly higher than what the car reports from the past, of course.

When you say this:
This mean that if you did 50 miles gently followed by 30 miles of crawling and stop starting the dash might say 45 mpg but the reality is when you fill up it may work out as low as 35 mpg because it's mixed the averages up for different scenario's you've driven.
Then you're right, but you have not understood why.
If you use ten tanks full of fuel doing economical driving, say at 6 l/100 km, and then use one tank at twice that, 12 l/100 km, then the trip computer will not report your consumption during the last tank, but since the last reset. If that was prior to using the first tank, then your average will be much better than 12 l/100 km, in spite of your last tank being there. It will be 6.3 l/100 km, since that's the average across all eleven tanks full.

If you want the trip computer to measure the average consumption across tanks, then you have to reset when you fill up. Which is completely redundant, since you can just as well calculate that yourself, if you just keep note of when you refuelled last, or reset your trip computer then, to get the distance driven.

[FreshAir]

Agree with the above. This is how it works.

[Pidgeonpost]

Originally Posted by apersson850

If you want the trip computer to measure the average consumption across tanks, then you have to reset when you fill up. Which is completely redundant, since you can just as well calculate that yourself, if you just keep note of when you refuelled last, or reset your trip computer then, to get the distance driven.

Which is what I do occasionally just to make sure my consumption hasn't gone completely haywire. Our annual mileage is only around 12k miles across 2 cars, mostly local runs, so it's just nice to check what I could be getting if I was able to enjoy the V70 on more motorway work - its natural habitat perhaps.

[apersson850]

Originally Posted by Pidgeonpost

Which is what I do occasionally just to make sure my consumption hasn't gone completely haywire.

You can do that quicker by keeping an eye on the instant fuel consumption while driving. You'll notice immediately if it starts deviating from what's normal, just as you after not too long will get yourself an opinion of what is just that, i.e. normal.

You don't have to wait for the next fill up.

I once noticed a faulty lambda-sond on my 854 Turbo just a kilometer or two from home. Instant fuel consumption displayed more than twice the normal, so obviously something was wrong.

[SnineT]

Originally Posted by apersson850

I'm not sorry, but you should be, since you are completely, utterly wrong.

The average fuel consumption is calculated by taking the sum of all fuel consumed since last reset divided by the distance driven since last reset. That's it. Nothing more to it.

You're talking old cars aren't you?

A modern car will record how much fuel is used at any given time, if it's giving 60mpg one minute but 34mpg the next it works an average out which is seldom exactly the same as what the fuel receipt says.

The way you're saying it is that if a builder was 2mm out at the wall he would still be 2mm out at the other wall where the reallty is he'd be more like 40mm out by the time he reached the other wall.

My S90 could show me 34 on the Sensus at the same time as 39 on the wheel button in the clocks and another different figure somewhere else all at the same time, which one should I have believed?

[apersson850]

Originally Posted by SnineT

You're talking old cars aren't you?

I'm talking about cars relevant to this section. But your S90 works the same.

A modern car will record how much fuel is used at any given time, if it's giving 60mpg one minute but 34mpg the next it works an average out which is seldom exactly the same as what the fuel receipt says.

Now you're mixing up instant and average fuel consumption. Instant is calculated as average across a very short time, like one second or so. It resets itself every second, if we say that's the time base.
The receipt tells you an amount you filled up. If you divide by driven distance since last filling you get the consumption. This is the same as you would get computed by the car if you reset the average at the last filling. Any difference is just due to measurement errors, not a difference in principle.

My S90 could show me 34 on the Sensus at the same time as 39 on the wheel button in the clocks and another different figure somewhere else all at the same time, which one should I have believed?

Of course it does. What you've not understood is that they are different values, since they represent different quantities.
The instant consumption is right now, sampled just the last second. It will be high if you floor it, or zero if you coast.
The average values in the Sensus display and on the clock are freguently different, since they are computed from different starting points. Unless you make certain to reset them at the same time.
If you reset one average, drive 1000 km with just the car, hitch up a caravan, reset the other average and drive another 1000 km, then you'll have two different average values.
The first one will show a lower consumption (9 l/100 km) and the second a higher (12 l/100 km), since the last only accounts for the part where you are towing, but the first also includes the segment where you were driving car only.

[SnineT]

Originally Posted by apersson850

I'm talking about cars relevant to this section. But your S90 works the same.Now you're mixing up instant and average fuel consumption. Instant is calculated as average across a very short time, like one second or so. It resets itself every second, if we say that's the time base.
The receipt tells you an amount you filled up. If you divide by driven distance since last filling you get the consumption. This is the same as you would get computed by the car if you reset the average at the last filling. Any difference is just due to measurement errors, not a difference in principle.
Of course it does. What you've not understood is that they are different values, since they represent different quantities.
The instant consumption is right now, sampled just the last second. It will be high if you floor it, or zero if you coast.
The average values in the Sensus display and on the clock are freguently different, since they are computed from different starting points. Unless you make certain to reset them at the same time.
If you reset one average, drive 1000 km with just the car, hitch up a caravan, reset the other average and drive another 1000 km, then you'll have two different average values.
The first one will show a lower consumption (9 l/100 km) and the second a higher (12 l/100 km), since the last only accounts for the part where you are towing, but the first also includes the segment where you were driving car only.

I'm not mixing it up I know about instant, long term etc, all I am trying to unsuccessfully make the point is that the reason the readout is often inaccurate to what the pump says is because it's constantly having to recalculate.

I know what you're saying and in the most it's true but if you get the chance to try a modern Volvo out look in the left pane at driver performance on the Sensus and it gives you the option to calculate 3 different distance recordings and reset each trip if so desired.

I used to work in garages and the mechanics cursed the day these calculators were introduced because Mrs X was getting bad mpg compared to Mrs Y who was lying to Mrs X anyway cos that's who she was back then, they'd see dozens of "broken" cars that just weren't doing as many mpg's as the bloke next door when the car was doing what it was physically able to do given 2L of suck that is 2L of suck in any 2L engine of the same model family. The customers used to forget their driving patterns, style and that some of them weren't able to let a clutch out fully.

I always get reasonably accurate figures because my usage is pretty well the same all the time, however if I do get out on tour and do some urban in between the readout is always wrong compared to the pump because at one point I was doing 60 and then 30 and then back to 60 mpg.

Where I think your idea is most accurate is if you read it after the car has stood switched off for a while then all the different points of monitoring get to collude with each other on start up and you get an updated figure.

[apersson850]

Originally Posted by SnineT

I'm not mixing it up I know about instant, long term etc, all I am trying to unsuccessfully make the point is that the reason the readout is often inaccurate to what the pump says is because it's constantly having to recalculate.

Of course there's a constant recalculation. As long as you drive, fuel is consumed and distance is covered. In each moment, the average fuel consumption is calculated as the amount of fuel consumed divided by the distance covered since the last reset.

I know what you're saying and in the most it's true but if you get the chance to try a modern Volvo out look in the left pane at driver performance on the Sensus and it gives you the option to calculate 3 different distance recordings and reset each trip if so desired.

Yes. So what? None of them will change you fuel consumption.

I used to work in garages and the mechanics cursed the day these calculators were introduced because Mrs X was getting bad mpg compared to Mrs Y who was lying to Mrs X anyway cos that's who she was back then, they'd see dozens of "broken" cars that just weren't doing as many mpg's as the bloke next door when the car was doing what it was physically able to do given 2L of suck that is 2L of suck in any 2L engine of the same model family. The customers used to forget their driving patterns, style and that some of them weren't able to let a clutch out fully.

That some drivers don't know how to drive economically has nothing to do with how the trip computer operates.

I always get reasonably accurate figures because my usage is pretty well the same all the time, however if I do get out on tour and do some urban in between the readout is always wrong compared to the pump because at one point I was doing 60 and then 30 and then back to 60 mpg.

At the pump you have to accumulate the total amount of fuel you have filled, at all occasions, since last reset, and the total distance you have driven, also since last reset, to get the accurate figure you can compare with the average shown by the trip computer.

Where I think your idea is most accurate is if you read it after the car has stood switched off for a while then all the different points of monitoring get to collude with each other on start up and you get an updated figure.

Are you kidding now? When the car is not used nothing happens to any of the calculations. They'll just take off where you left them, regardless of whether that was two minutes or two years ago.

[SnineT]

Okay I'm done with it have it your way.

The thread was about all aspects of consumption not just the trip reader and that's why I digressed to how people drive affects what 2 different people will achieve in the same car etc, I still stand by how the thing works and it doesn't or is unable to give you anything more than a rough idea, I find I get pretty near my readout but others won't and that's because of their patterns being different to mine.