I think the 'miles left' digital read out is the only useful thing on the trip computer itself. On my old motor (a Ford, so they can produce good ones if they want to), if you kept your style of driving sensible, the reading went down at a sensible rate, and was the same when you switched the car back on later.
This one, double the price of my old motor, has a mind of its own and magics juice out of thin air while it's been sat on my drive at home. It drops like a stone, no matter how fast or slow you drive, motorway or side street, and has really left a sour taste in my mouth.
The CAD Engineer at work says perhaps it's because its only 1/4 full, and behaves more 'realistically' when the tank is full. So I'm gonna do the same test as in my OP, but with a full tank, and see how fast it goes down.