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Trip computers tend to be inaccurate for the following reason. Speedometers over-read by anything up to 8 percent or so. Therefore if you stick at 60 mph for 1 hour you may have only travelled 55 miles or so. The trip computer takes its mileage from the speedo so it says for example that youve used 1.5 gallons to travel 60 miles ( 40mpg) when in fact youve travelled 55 miles using 1.5 gallons or 36.6 mpg. Check out your speedo with a good sat nav and you`ll see.

My Vectra CDTi says its doing 82mph on the French motorway system ( where you can put the cruise control on for long periods unlike here) when the sat nav s saying 75!

Actually, having said that trip computers aren't much good, mine seems to be turning out pretty accurate - over the last two tanks, it has matched the calculation produced by the Fuelly website...

...which is sort of annoying because when I see it says 17mpg, I know it's telling the truth...