I've checked the fuel comp for accuracy by working out the MPG the "old-fashioned way" and it's consistantly about 0.2mpg out - which is pretty accurate as far as I'm concerned (especially when the fuel comp in my old cavalier used to be about 3-4mpg out each time I worked it out).
I did a 750ish mile trip to France and back last year just after I bought the Vectra. When I got home the average mpg was showing 36.5 mpg.
On my normal week I used to regularly see 31-33mpg average for a tankful of fuel. Now, because the heap of poo is running like a bag of shite, I'm lucky to hit 29mpg 🙁
Hopefully that will all change once I can find out what the hell is making my car run like crud......