I'm not at sea level, ~3450'. My 55 was much too busy sounding with the APC 11x5. 12x6 was too much load. Sweet prop is the APC 12x5 on my 4*40. Low idle, instant transition, fast, unending fast vertical. Doesn't use much fuel either, but I'm not at full throttle all the time. If memory serves, ~ 12,400 flying rpm.