Overall mpg display being fooled?

[wildone63]

Last November I had to have the fuel system drained and flushed on my 2008 Corsa 1.3 diesel due to putting contaminated diesel in the tank.
Since then the overall mpg figure on the centre dash display has been getting steadily worse going down from a reading of 55.7mpg to a current reading of 54.0 mpg over about 1500 miles.
Approx 25 litres was drained out of the tank so iam wondering if the 'brain',ecu, computer' or whatever it is that calculates/provides the reading for the overall mpg figure has somehow been fooled into giving a false overall mpg reading? IE - the car has had 25 litres of fuel gone from the tank but didn't clock up any mileage to account for its disappearence ,so it appears to the computer that the car has used 25 litres more fuel than normal over the last 1500 miles.
Any thoughts?

[Scrump]

More likely to be due to the lower temperatures in December and January.

[Steamer]

Scrump said:

More likely to be due to the lower temperatures in December and January.

I always notice about a 1.5mpg drop on the onboard computer during the 'diesel winter mix months'.

[996TT02]

Don't imagine that tank fuel level is used for avg MPG it would be all over the place and unusable till you have a few tankfuls.

Mine (petrol) counts injector cycles and even gives a simulated tank level readout based on them. With a diesel it might count something else.

[LuS1fer]

No. If you reset it, it will start again.
The computer should take your fuel gauge level and predict the range based on your average.
I wouyldn't take them to be that accurate anyway, especially not back in 2008.

[wildone63]

Scrump said:

More likely to be due to the lower temperatures in December and January.

Really? ive never heard of any vehicle whose fuel consumption goes up in the winter.

[Chris32345]

Mpg calculator is based of of distance covered by counting avarage rpm of the wheels same as the Speedo does
And flow rate through the injectors and works our how much fuel it has injected from those two to give you a mpg readout


All though that small of. A distance could be down to anything

[Jimmy Recard]

wildone63 said:

Really? ive never heard of any vehicle whose fuel consumption goes up in the winter.

Very normal. Cold engines to warm up and thicker air for a start

[rainmakerraw]

wildone63 said:

Really? ive never heard of any vehicle whose fuel consumption goes up in the winter.

They all do. Winterised diesel fuel, cold air, denser air, longer warm-up times, wet roads... the list goes on. You've lost a whole 1.7mpg over three winter months. If anything you've done well it's not more. The dodgy diesel in November will have nothing to do with it. You can vary your consumption by 1.7mpg with 1mm difference in throttle position, different use of brakes, an extra red light during a day, tyre wear and pressure differences, an extra bag in the boot (or weight on your belly)... it's nothing.

[LuS1fer]

Have a look at the instant mpg when cold, you'll get maybe 12 mpg in the first mile, when cold.

[wiliferus]

Yup, my old D5 Volvo loses about 3-4mpg during the colder months.

[wildone63]

rainmakerraw said:

They all do. Winterised diesel fuel, cold air, denser air, longer warm-up times, wet roads... the list goes on. You've lost a whole 1.7mpg over three winter months. If anything you've done well it's not more. The dodgy diesel in November will have nothing to do with it. You can vary your consumption by 1.7mpg with 1mm difference in throttle position, different use of brakes, an extra red light during a day, tyre wear and pressure differences, an extra bag in the boot (or weight on your belly)... it's nothing.

Yeah I know many factors can have an effect on fuel consumption,but it just seems a bit much of
a coincidence that the fuel consumption started going up straight after the fuel system was drained
and flushed
The car also had a replacement ecu fitted around the same time but I cant see that having anything
to do with it.

[Flumpo]

wildone63 said:

Scrump said:

More likely to be due to the lower temperatures in December and January.

Really? ive never heard of any vehicle whose fuel consumption goes up in the winter.

Even my wife comments on this, and Her total knowledge of cars is that she has a blue petrol fiesta.

My diesel had a dizzy high of 57mpg in the summer, now it won’t go over 52. Same journey same speed.

[rainmakerraw]

wildone63 said:

Yeah I know many factors can have an effect on fuel consumption,but it just seems a bit much of
a coincidence that the fuel consumption started going up straight after the fuel system was drained
and flushed
The car also had a replacement ecu fitted around the same time but I cant see that having anything
to do with it.

A new ECU (with potentially entirely different code on board) would have far more potential to affect fuelling than empting the tank and filling it back up again... Regardless, the answer remains 'winter'.

[swagmeister]

1.7mpg requires posting on a forum ? wow

[wildone63]

swagmeister said:

1.7mpg requires posting on a forum ? wow

Is there any need for the sarcasm?
Iam not losing any sleep over the loss of 1.7mpg (and still rising)
,I was just trying to get some suggestions as to the cause of it,and
why it coincides with the work I had done to the car