Well I do runs between Reading and Suffolk every weekend, so my fuel costs soon add up! I have a 1.2 Clio Dynamique.
The best I ever had was a couple of weeks ago when I did everything I could to get the mpg up, pumped my tyres up, had no weight in the car etc. I cruised at 60mph with the lorries for 2 and a half hours but got 65.5mpg!
One night I drove down though I think I had a batch of aweful fuel and I only got 37mpg, and that was trying to save petrol!!
I find at 70mph I can get about 46mpg on Shell fuel.
At 80mph about 38mpg
At 90mph 34mpg.
And having to drive on the M4, M25 and A12, I usually come across a traffic jam anyway, so it usually makes little difference to my journey time!