I think bikes are more prone to the environment than cars. I live in rural Suffolk, UK and rarely get above 60mph. I average 82mpg (imperial) on my 500. Recently I've had to do a few 50mile runs on a East-West dual-carriageway travelling at 70+ mph with the return journey battling an Easterly wind. My mpg dropped to 70. My 2litre diesel car always returns 58mpg whatever I do.