not sure whom you mean by "they".
Fuel flow (at a constant Mach) depends on temperature. If it's warmer than ISA, you need less power to maintain the same speed, so therefore the fuel flow decreases, and vice versa.
Anyhow, if I need to do LRC on the XP, I usually set power to maintain M.70 and the FF will be somewhere in the range of 1300, if the temperature is on your side you might be lucky to get it down to about 1250. Try to cruise as high as possible (check the tables for maximum cruise weight in your QRH).
Good luck and safe flying ;-)