Just to add to this thread - my MPG on 220 is now down to around 27 or 28 but maybe because it is a 40MPH limit and 4th/5th gear thing.
I have noticed it is getting worse with age. I am now on 41k and very surprised how bad it can be.
On a motoway now I drive at maybe 65-70PH in 6th gear so that the rev counter is below 1500 - I have no power to overtake or anything and yet my computer MPG shows only around 35 MPG! I'm not sure what else can be done to improve the MPG but I hardly touch the accelerator there is no weight in the car and it absoultely nowhere near the reported 50 or so MPG when I bought it!