The dash meter tends to enhance the true fuel economy by 5 - 10 per cent. Brim to brim calculations are more accurate, but to be totally correct you'll need to account for correction of the odometer which, typically, will over-read by 2 - 6 per cent.

www.honestjohn.co.uk/realmpg tells you what real drivers are achieving in real cars on real roads, but mostly on the basis of their dash meters.