I've done a few long and fast trips over the last few weeks.
60mile trip, ~40miles pottering, 420mile trip quite alot of the latter at 90ish on 51l of esso premium diesel (forget what its called)
400mile trip, 30pottering, 120 motorway and into london on 57l V-power
120pottering, 250 trip @ 70, 250 trip @ 80 - 58l esso premium diesel
Despite regularly getting the computer to read 60mpg, albeit the average to a tank rarely over 51-52, I very rarely see more than 45 when worked out at the pump, and its 41-42 if using regular diesel. I haven't had the opportunity to try a long trip at <65mph which I think is what is needed to get decent real figures. This is a 2009 ST 143 SE with 135k on the clock.
It normally takes 10-20miles before the average mpg starts to get up to something reasonable so wonder if I have a lazy stat which won't be helping, I stupidly forgot to ask them to change it when they did the water pump at cambelt time in April. I've just ordered VCDS so I'll log the water temp and see what's what.