They are only rough estimates using existing vehicles, but engine losses are pretty constant, and if you take the 'power to wheels' group, wind resistance is always the largest factor (especially in the 'highway' cycle, which is sort the best one for your concern).
Choosing gears to make the engine most efficient at a higher speed will increase that contribution (without really changing any of the others much; I guess there are a bunch of critical points where the different fuel losses cross each other, and they aren't necessarily straight lines).
http://www.fueleconomy.gov/feg/atv.shtml
They are only rough estimates using existing vehicles, but engine losses are pretty constant, and if you take the 'power to wheels' group, wind resistance is always the largest factor (especially in the 'highway' cycle, which is sort the best one for your concern).
Choosing gears to make the engine most efficient at a higher speed will increase that contribution (without really changing any of the others much; I guess there are a bunch of critical points where the different fuel losses cross each other, and they aren't necessarily straight lines).