I recently had to get a new lower unit due to destroyed gears. I had a smaller 4.25 size lower unit prior with a 24p tempest plus II. I used to be able to turn 5700 rpm and high 50's on my 1990 ranger 482 with 95' 150 xr6. My new Lower unit is a larger gear case (4.75) and using the same prop. Now I can only turn 5100 rpm and max out at 52mph. I know lugging these motors is not good and I believe that's what he m doing The motor runs fine but I would love some suggestions/information on what may have happened. Is it that the prop is too small?, diameter not correct? Too much exhaust escaping? Thanks in advance guys.