Thats a pretty broad question that there is no one answer for. Water condition, outside tempurature, prop pitch and amount of trim can make the top speed vary greatly. With my 3.0 under the most ideal conditions I was able to hit 48 once. Generaly, 38 to 40 was the norm but running like that sucked the gas. So I guess my answer would be if run hard, around 40. For cruising, keep it to 25 to 30.
Id say 40 is a good guess. My old 19' Regal (1987) with a cuddy and a 180 hp merc would go 43 mph on the GPS. Dont trust the boats speedo if it has one. Use the gps from your car (or borrow one) for a true reading. 38 - 40 I'd say should be close.
