It lies! Mk3 8v 2.0 here, and it runs like crap 
If you reset your trip computer while you are driving, it will jump up massively. I did it one time and hit 99.9MPG for a couple of miles (even got a pic somewhere I think). Oh, and I wasn't driving particularly sensibly on the dual carriage way :/
Well of course it will jump massively! Before being reset it's calculated the whole journey including the period it was running cold, and the journey to the dual carriageway, including time spent stood still (for which your real time MPG should be 0). After it's reset the whole calculation is based on a warm engine, holding a steady cruising speed in 5th gear.
99.9 is more than plausible - If you were going downhill at the time (even a long gentle slope), had reset the MFA at the top of the hill and were cruising in gear with no throttle, then you would be using no fuel as the ECU will shut off supply to the engine.
45.5 was the best sustained average I got in my MK2 8v, travelling at a steady 70mph on the speedo. I've never seen above 35mpg in my 16v 
Actually, I was doing..um..motorway speeds
on the level, and up hill. I will admit it was an unusual fluke, usually it will go to about 45-50mpg if I do the same.
Besides, I would want my reading to be current, not an average over the entire journey.
