Well, if 15x4 was too thrusty (not sure what fuel economy has to do withit, but you were probably flying at a higher throttle setting to keep speed up with the low pitch), and a 14x8 was too fast, then a 14x or 15x 6 would seem to split the difference.... Buy one of each, fly'em both and decide. The 14 will turn better RPM's, and thus have better speed. The thrust should be similar betweeen the 2 with the rpm difference. If the 14x6 isn't enough prop, try the 15x, and trim the tips till you get the performance you want.

J