I always "calibrate" my trip computer against actual calculated consumption - cars seem to be getting worse, my 2000 Vauxhall was 3% optimistic but it's range to empty was very accurate especially at low fuel levels! My 2006 Subaru was 6% optimistic and my 2011 Hyundai is 9% ecstatic and the range to empty is way out on both of these.

Iv'e just been reading the mpg section of Honest John. Don't take to much notice of trip computers before you have checked them out by noting down the TOTAL mileometer reading, filling the fuel tank up, doing the miles, topping the fuel tank up again, then deducting the last mileage reading from the present reading, then dividing the answer by the gallons used. Do this several times to check out the trip meter fully. My trip computer reads 5 mpg more than I actually get. 2.0 ltr, 130ps, Mk 3, Tdci, Mondeo, owner.

Could not agree more. Our current car is pretty accurate but had one once that was 8mpg over the truth.

One other thing I never considered until we changed cars last year. Twice a year we visit Scotland, same route year on year. In the Old VW Golf it was 452 miles door to door, when we swapped cars the lodge got closer, now its only 414 miles. That difference makes a huge difference to mpg calculations. The Golf always did a calculated for the week of about 60 mpg which we thought was incredible. New car, Kia Ceed, only did 55 mpg thus a bit dissapointed. But if you recalculate using the mileage recorded by the Golf it becomes 60 mpg as if by magic and we are happy again.

Checked distance with the Garmin earlier this year, it showed it as 430 miles and AA Autoroute agreed. Recalculate both cars using that mileage and they both average 57 mpg.

If you have a Sat Nav its worth checking the accuarcy of the odometer, you may be getting more mpg than you think. Then again you could be getting less.