I'm sure this question has come by here before but I have searched for it and couldn't find it so here it goes: going down from say 70 mph will get your FE up. But suppose I just put the last available gas on earth in my tank and I would want to drive as far as I could with that last gas tank how fast would I have to go? Is 10 mph better than 40 mph? or is it the lowest speed in which you can drive in 5th gear?
(Oh and I am also assuming the roads would not be full with cars that wouldn't be able to drive any further
).
McPatrick 
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