My commute to work I set the cruise at 65 though the speed limit is 70. Over a couple of hill (nothing big or severe grades) I've noticed my Fit will downshift when going up when the cruise is set at 65 but it doesn't if I'm going 70. So my question is which way is better according to fuel mileage?

 

Maintaning 70 mph in 5th is more economical than 65 in 4th. Next time check change in rpm. Rpm is representative of mpg at those speeds. The slight increase in gas consuption in 5th due to 70 vs 65 is outweighed by the difference in rpm.