I drive mainly longish (400mls+) distances over a couple of days, and the on-board computer consistently tells me I'm getting close to 70mpg. Yes, I have the 1.5 diesel. But, when I come to fill the tank the real mpg is about 10% lower. Is this a common problem? Is there anything that can be done to adjust the readings. Whilst the high numbers look impressive, I'd rather they were right!