the 24V granny sees between 13 and 17 mpg in and around town
on the motorway it sees around the 33 mpg mark
the trip computer is just about spot on give or take a few 10ths of a ile lower than the trip meter says in the speedo cluster
with a mixture of motorway and town it's more along the lines of about 22 mpg

i did a trip back from nottingham once though, in which i recorded a 77.8mpg full tank to full tank (or whatever you figure it out as with 130 miles coverd in 7.88 litres) in my dohc sierra with the ultra low diff and slip streaming a lorry at 55 mph

the same trick doesn't seem to yeild as good a result in the granada though, but you do see upto about 45 mpg on the trip computer as opposed to about 33 mpg when not tailing a lorry

also had a few hire cars, a fiat panda with an auto box returning more than 50 mpg in town adn another one i can't quite remember the name of returning 66

on the other hand, i was lucky to see 160 from the 60 litre tank in the estate and once did 18 miles and used 22 litres of fuel