I do very few miles in my doblo diesel van, lots of stop/starts which averages about 42mpg over about 600 miles a month. It does 55mpg on a run, but that is a very rare occurance.
The family car is a 7 yr old diesel pug estate and has a trip computer thing like the van. I went to Cardiff and back this weekend, five - up and being a round trip, door to door of 81 miles on m-way I thought I'd set the cruise control to 60/65, slip stream coaches (safely, not too close) and see what I could get mpg wise.
Including getting to Cardiff city centre and parking up in an almost full car park it took about 50 mins each way and averaged 62.4mpg!
Went to Alton Towers with a tailwind week before last, 4 up, cruised at 70 on motorway and got 60.1 mpg going and 55.7 mpg coming back (into wind).
So... drive like an old ....... and save the dosh.
(I've also started mixing in a bit of legal biomix which is very cheap)
Enthusiastic fuel burning is now the sole reserve of my motorbike!