It's a similar issue with helicopters, which are in effect turbo-props, where quite often the major factor is headwind / tailwind. On one type I flew for offshore SAR, we carried out trials to gain more data than the Flight Manuals provided. Our concern wasn't so much getting there, but getting back to landfall again. We found the best way to minimise fuel used was a cruise climb to altitude (less than 10,000 feet in our case because we were unpressurised) and then a slightly reduced cruise speed if there was a tailwind. If there was a headwind, reducing the cruise speed was pointless because it was offset by a longer airborne time. There was a page in the FMS that showed I specific fuel consumption, i.e. Fuel being burned per mile, which was a big help. Obviously, mainly because of the wind factor, it varied from flight to flight.