Huge Difference Between Onboard Mpg And Reality

[KAR]

Galaxy 2010 2.0 diesel Zetec powershift. I've a very light footed driver and seldom use air-con etc. In my previous cars have got close to the official MPG. I reset the trip computer on every tank full and note the actual fuel used and compare the trip computer MPG for the tank compared to the actual fuel used. On every occasion I find the trip computer is about 5-10 mpg better than the actual fuel. For example the best ever MPG for a tank was 43.1 according to trip computer but the actual fuel used worked out at 36.8. Why is there such a difference between the two? What average MPG do you achieve?

[KAR]

Just an update with the last two tank fills:

Total miles 636.2, computer average MPG 49.8, litres to fill up 68.14, actual MPG 42.4, difference 7.4 15%

Total miles 543.7, computer average MPG 50.1, litres to fill up 61.84, actual MPG 40.0, difference 10.1 20%

Is the difference between your real MPG and what the computer reports so large?

[btmaldon]

Assuming you also zero the average MPG when you reset the miles to zero when you fill up, I cant explain it. My Focus is pretty accurate.

[stevegtuk]

not sure about galaxys but its well known that mondeos can be out with their mpg calculations, in some cases I have heard from, the difference between dash and manual working out has been as much as 10-12mpg. mine dash says about 5-6mpg more than it actualy does.