Craig I would suggest you aren't far off the mark there. Each car will vary but I think around 50mph is where it's most frugal.
Incidentally I drove 200 miles to sheffield yesterday and back! Long drive but I decided to take my time, accelerate gently and keep it to 70 exactly, very little change in speed other than for overtaking. I am now sat at 35.4 mpg, not too shoddy at all, and on target for about 340 miles out of a tank allowing for computer inaccuracies, much better than usual!
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