here is a thread i started awhile ago wanting to know speed differences. maybe that'll help. someone with a 120 responded in it.
I guess that would be the 120 hp? If a 120 would push one that fast,a 165 would be around 50??
all i can say is what i've observed with mine. max speed i ever got out of mine is 44mph @ 4400 rpm. which is 100 rpm faster that wot spec. i've never seen close to 50 out of mine. when ever i do a speed run my 1st statement holds true, always, assuming all is well with my boat. my speeds are gps and as far as i'm concerned, a fact about the speed (41-43mph) of my nineteen. original drive (1.65:1) and engine.
the varaitons in my speed i think are about air temp, humidity. when both are high, i go slower. also when i do the speed run. 1st run of the day or wait till end of day after the engine is really warmed (heat saturated) up. again, slower speeds when heat saturated.