I took the E500 to London and back yesterday (round trip from Nottingham ~250 miles). Annoyingly I had a bit of a wheel wobble above about 65mph with my newly fitted winter wheels - first time I'd had them up to speed, and of course I'll be visiting the tyre fitters today to get them re-balanced. But it meant I chose to use the speed limiter to stay at about 63mph - just below the point where the wobble would start - and that, plus interminable stretches of the M1 being restricted to 50mph, meant that I got the best fuel economy I'm ever likely to get from this car.
33mpg on the way there, and 32 mpg on the way back. Check my working, but I make that 65mpg