This isn't going to be the best by any means but I recently did a 1,520 mile trip in my 40d, with 3 passengers and myself plus bags etc averaging 53.7mph, towing a 2 tonne 12m long trailer and still getting 23.5mpg
It would have been higher than 25mpg except I had to make up a bit of time to catch the eurotunnel on the way back.