sorry to be so negative
but if it helps we have just returned from a 2 week break in Devon & Cornwall -700 m/w mls / 250 non m/w mls and purposely keept speed down to 56 / 60 mph and returned 28.5 mpg
normaly get about 27 mpg but also travel at 65 / 70 mph
but before fitting a roofbox it returned 31 mpg from a 2.8 iveco td -4.2 ton - without the big head bump
terry