Purely coincidentally I took a day trip over to Kimmeridge in dorset yesterday - so it was 120 miles each way of mixed Dual,Mway and country roads !
When we left sunny sussex the avg mpg was reading
54.2 and ambient temp approx 28deg.
After 100 miles of driving - with the ambient now reading 32 deg and having used the aircon continuously - the mpg reading had plummeted to
53.2 
.
Cruising where poss at 75 and using cruise control (not exactly subtle on the throttle
) as req !
car = S hitroen C3 picasso 1.6 HDI - not exactly aerodynamic LOL