I have a 88 Chris craft limited with a 260 in it,alot bigger boat and with a 21p Mirage and a holley 650 and aluminum intake,it goes 48.9 on gps and that is the only way to get a true mph.I have never seen a boat speedo read accurate.I used to have a 20' sea ray with a 260 and it went 51 on gps with a 21p prop and stock engine,I would say you should be around that as the boats should weigh around the same,also with a similar hull desighn,but water conditions can reflect on that.