Its those short slow journeys.. I've been bored and playing with the Instant MPG feature on the ZT Auto.
Zero it when doing about 25-30 on a cold morning after starting off and you will be lucky to see 20mpg, I get around 15-17 .
Floor it off the start and you could see 8-10mpg !!
But then again cruising on the A road @ around 60 with light foot it will give nearly 70mpg .
So average is some where in the middle
I did try very hard one evening coming home from work, 15 miles, and from a stationary zero'ed IPK (Instrument pack (IPK from the German))
I pussy footed on 8 miles of carriageway and 7 miles of urban country roads and got home with 52 mpg on the dash !! and no real time lost from normal so it can be done ..