Boss, keep us posted. Something is definitely amiss. You should be getting more rpms and speed. I had a 272 with a 502mag that would run 70speedo and 68gps. You should be close to 65speedo for sure.

One more thing, those speeds were with a 23 Mirage. I switched to a 24 Bravo which gave me much better performance all around, but I did loose a bit off the top end.