Journey type, length and the time of year are big factors.
Shorter journeys at the moment will use a lot more fuel, as the warm up stage will be longer to compensate for the lower temperatures.
I used to do the best part of 100 miles a day (50 miles to work). The MFA would say 42mpg, which worked out to be about 38 mpg, which was pretty good for a constant 70 with occasional bursts of over-taking.
I now do about 250 miles a week. Most of my journeys are to work (6 miles round trip) and then to Uni 3 days a week (60 mile round trip)
The MFA now says about 34 MPG, which is a bout 30 mpg in real terms.
I'm sure if I went back to longer journeys the mpg would go back up.