40 MPG !? HOW!? I've done several stints around the M25 stuck doing 50 and the highest I've NEVER seen my car in 3 years of ownership go above 37.X (something).
Actually I don't know. It was a trip to Heathrow, through the long roadworks (50mph) at the top
of the M25. Together with normal commuting resulted in 40mpg (measured using the brim/odometer method).
I've seen plenty of other posters claiming even higher MPG, but my max is 40.
My car is manual, running 17"s and I always keep a close watch on tyre pressures.