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Headwind and tailwind have a reasonable effect, road surface (watch the mpg dive on the concrete M180), uphill and downhill (15 miles to work - 55 mpg there, 41 back all at the same speed) but most of all speed. I once had to go really slowly (going to an airport, too early, with 4 year old in the back so wanted to reduce airport time) so I slipped in with the slow lorries at 56 mph and low and behold 62 mpg over many many miles.
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