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Wildly inaccurate MPG

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Old Sep 8th, 2017, 08:23   #1
LoudPedal
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Default Wildly inaccurate MPG

Hi all, my first post so apologies if it's in the wrong section!

I've been surprised at the rate my 03 V70 D5 AWD geartronic has been using fuel. I measured economy between the last 3 fill ups (reset trip and mpg) and found that the trip computer was telling me 46.5mpg but the calculated economy from the pumps and odometer was just 37! This was consistent between fill ups.

Does anyone have any suggestions what might cause the indicated mpg to be 25% higher than actual? There's no sign of Diesel on the driveway so I presume it's all making its way into the engine somehow.

I'm thinking that the indicated figure should be achievable with the roads I drive and my driving style. I do my "spirited" driving in my other car (a highly tuned N/A mx5) so driving this one is all about fuel economy. My last practical car (02 Toyota Avensis 2.0D) managed around 63mpg actual measured economy so I'm pretty shocked how thirsty this one is!

Has anyone else experienced similar?
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Old Sep 8th, 2017, 15:57   #2
GrahamBrown1
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Pretty normal fuel consumption for an auto d5 of that era in my experience. The dim display is never that accurate.
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Old Sep 8th, 2017, 23:07   #3
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Yeah, it's not too horrendous but it's 40-60mph cruising almost the whole way from home to work so I was hoping for a little better... My last car managed 15mpg higher than quoted on the same route which would put the trip computer mpg about right.

10% out I could deal with bit 25% out makes me feel like something isn't right...
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Old Sep 9th, 2017, 09:22   #4
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I think a lot of the internet hype about the astonishing frugality of the Euro 3 D5 engine comes from the utter lies the trip computer tells. On my 2004 V70 T5 it always over reported by 20% or more too.

Sensibly, a 1700KG car with an automatic gearbox and AWD system with what is not exactly a state of the art diesel engine cannot be expected to do much better than 38mpg.

For comparison, my C70 is about the same weight, has a newer revision of the engine, and is FWD and a manual and that averages 43mpg if I'm gentle. Lots of motorway driving.

I'd expect to do no better than you in our brand new manual XC60 D5.
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Old Sep 9th, 2017, 09:41   #5
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Good to know I'm not the only one who's noticed this! Sounds like perhaps this is normal behavior for these cars.
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Old Sep 9th, 2017, 14:25   #6
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55-60 mpg measured at the pump after a motorway cruise euro 3 d5 manual.
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Old Sep 9th, 2017, 16:54   #7
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What fuel are you using?

I have a Euro IV 185BHP D5 auto and whilst mines not that badly out, it seems to be closer with Shell/V-Power then when I used Tesco Diesel for a bit.
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Old Sep 9th, 2017, 19:29   #8
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Quote:
Originally Posted by LoudPedal View Post
Yeah, it's not too horrendous but it's 40-60mph cruising almost the whole way from home to work so I was hoping for a little better... My last car managed 15mpg higher than quoted on the same route which would put the trip computer mpg about right.

10% out I could deal with bit 25% out makes me feel like something isn't right...
I would get the engine and instrument panel software updated first ... do you zero the average speed and fuel consumption before starting your calculations ? Are the tyre sizes as standard ?
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Old Sep 9th, 2017, 21:18   #9
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Quote:
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What fuel are you using?

I have a Euro IV 185BHP D5 auto and whilst mines not that badly out, it seems to be closer with Shell/V-Power then when I used Tesco Diesel for a bit.
Interesting... How much difference do you get in (real) mpg between the two? I'm using Tesco at the moment but could be persuaded to try Shell if it was going to work out more economical overall.

It's funny that it would make a difference since the ECU will be doing its mpg calculations based on how long the injectors are open for rather than any sort of power measurement. The only way I can see the fuel brand making a difference to the accuracy is if the shell fuel was more viscous so less flow for the same injector time.

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I would get the engine and instrument panel software updated first ... do you zero the average speed and fuel consumption before starting your calculations ? Are the tyre sizes as standard ?
How would you go about updating the engine and instrument panel software? Is it a "pay the dealer lots of money" kind of job?

Yes, I have been zeroing the average speed, mpg, fuel consumption and trip when doing the calculations so it should be a fair comparison. Tyre size is standard and I'd expect that to affect the odometer and mpg numbers equally.

Last edited by LoudPedal; Sep 9th, 2017 at 21:24.
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Old Sep 9th, 2017, 21:59   #10
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Quote:
Originally Posted by LoudPedal View Post
Interesting... How much difference do you get in (real) mpg between the two? I'm using Tesco at the moment but could be persuaded to try Shell if it was going to work out more economical overall.

It's funny that it would make a difference since the ECU will be doing its mpg calculations based on how long the injectors are open for rather than any sort of power measurement. The only way I can see the fuel brand making a difference to the accuracy is if the shell fuel was more viscous so less flow for the same injector time.



How would you go about updating the engine and instrument panel software? Is it a "pay the dealer lots of money" kind of job?

Yes, I have been zeroing the average speed, mpg, fuel consumption and trip when doing the calculations so it should be a fair comparison. Tyre size is standard and I'd expect that to affect the odometer and mpg numbers equally.
The Engine ECU and others would be updated at your dealer service free , when did the car last visit a dealer for a service ? The DIM may or may not be updated depending on the importance of the other updates needed at the time . I have not come across fuel computers so far out before , they are usually pretty close ....
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