I agree with you. I wouldnt doubt your bike got better mpg at 75 in 6th as apposed to 6th at 55 because as you said it was working at optimal rpm of the engine to make power. It probably uses a tiny bit more gas, but you are also covering more ground faster, Giving you better mpg so the little extra gas you use doesnt out weigh the ground you covered. Of course it probably would have even gotten better mpg working in the same rpm range say in 5th going 45, but this isnt the case because that is too slow for the highway. He and I did not claim faster in general = better mpg. Just at certain speeds at a certain rpm. I am no expert, but this is just my thinking on it.