kind of a loaded question
in real mph I'd say 40 is on the low low side
things that need to be understood
what did you use for a speedo - gps or radar - if not you're only guessing
what is the engine rpm @ WOT - I only trust my digital tach
fuel tank full or empty?
water glass calm or a little choppy?
how many on board 2 or 3
what else is under the dash , boom, skis, wakeboards etc
typical Advance speeds with a decent tune should fall between 43 & 45
my old 18 would see 43.3 - 43. 8
my Chevy powered advances saw 44.2 - 45.8
if your turning a real 4300 - 4500 rpm w/ 13x13 you should be 42-44 mph
Dan T