Disappointed with petrol consumption. Admittedly we use car for short journeys, 5-7 miles. Onboard computer gives petrol consumption 38.5 mpg but based on fuel used ,tank full to tank full, and miles driven works out to be 32 mpg. This is average for last 1000 miles. Can onboard computer be really -20% in error or is there likely to be other problem such as a fuel leakage? Car is going for first annual service next week ( 6500 miles driven)and any thoughts you may have may help a diagnosis. Your advice would be much appreciated, thanks David France