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Wildly inaccurate MPG

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Old Sep 10th, 2017, 07:22   #11
tt82
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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.
IIRC Using Shell the Car was generally optimistic by 2-3mpg, with Tesco it was usually a difference of around 5mpg.

All the stats are available on my Fuelly profile in you want to have a look.
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Old Sep 10th, 2017, 09:45   #12
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Ditto (and I stopped using supermarket fuel nearly the moment after I started - did not like slightly different engine clatter) - I use Shell V-Power or Total Excellium Diesel only
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Old Sep 14th, 2017, 17:35   #13
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I once had a BMW diesel with wildly optimistic mpg readings that had been given a fairly crude remap - primarily an increase in fuel rail pressure. I always assumed the two things were linked - has your car been remapped perhaps ?
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Old Sep 14th, 2017, 18:15   #14
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Originally Posted by poiuytre111 View Post
I once had a BMW diesel with wildly optimistic mpg readings that had been given a fairly crude remap - primarily an increase in fuel rail pressure. I always assumed the two things were linked - has your car been remapped perhaps ?
Shouldn't be

The engine control unit opens the injectors for xx microseconds

Given known fuel rail pressure and injector size and injector opening time, the amount of fuel used can be measured exactly. Regardless of remap.

Combine exact fuel usage with exact distance travelled and you can calculate the MPG (or km/100L)

I think it may be more a sampling size issue

At any rate, it is entirely software driven and no adjustment is possible....
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Old Sep 15th, 2017, 19:41   #15
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Doesn't matter what fuel we use the display reckons 38-40 mpg whereas in reality we get 32mpg in our manual xc90 which although I figure isn't too bad it does indicate that the makers inhabit some kind of parallel universe.

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Old Sep 16th, 2017, 07:42   #16
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V70 Tdi, run from Cornwall to Gatwick then on to Eastbourne, return the following day. Good steady limit plus speeds with little stop/start. Full tank to start and full tank when home, 51 on comp. 51 actual. Only time they ever matched. Normally comp. 2/3 optimistic and all on Morrisons fuel.
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Old Sep 26th, 2017, 00:42   #17
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I have a 163 D5 on 255k, most of which I've driven.
Mine is not AWD, and is manual.
Currently the (optimistic) trip computer reads 53.4 mpg. I expect when I calculate it brim-brim it'll be about 45-48 mpg, which is typical for my car.

BUT... I've found that my best MPG is consistently from V-Power and the worst from Tesco diesel.
When I was doing 200 miles per day on a commute, I ran various fuels to compare them. I worked back to a cost per mile.
For me, Tesco worked out the most expensive per mile, and v-power the best. Other supermarket fuel was marginally better. BP Ultimate was similar to v-power.
On the first tank of premium diesel after poor fuel, mpg will be better.
On the second tank it will be better still - and will remain similar while you use the same fuel.

Your auto box and AWD will reduce the MPG considerably compared to my car however.
I also tend to sit in lane 1 on the motorway if in no rush, it's much more economic at ~60 mpg than 70+.

With a combination of decent fuel and infrequent 'foot to the floor' 700-750 miles per (70L) tank is easily achievable.
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Old Sep 26th, 2017, 14:52   #18
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Previous 180hp D5 manual would almost always get MORE mpg when calculated brim-to-brim against mileage, than the computer suggested with reset at each fill-up. Current D3 Geartronic gets almost exactly what the computer says in reality. Both cars were/are used mainly on 40-60mph long runs (15 miles or more with each start-up), light foot on the loud pedal and no more than 65mph on cruise control on longer motorway runs. D3 gets 43-45mpg, D5 would get 46-48mpg. Shell standard diesel throughout.
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