The 48 mph number we achieved in the Chaparral was was statute mph, NOT nautical mph. So translate that to 41-42 kts. That was the speed showing on the GPS, trimmed out WOT in a moderate chop to choppy conditions in Choctawhatchee Bay. We reached 45 mph easily, and then played with the drive trim, and she gradually eased up to a max speed of 48 statute miles per hour.
It makes no difference whether you believe it or not. Not my boat, not my motor: I have no dog in this hunt. It was just an interesting day on the water, which is generally better than an interesting day on the shore.
On the other hand, MY 1987 Chaparral 198F with its 1988 Johnson 140 Looper can reach a WOT speed of 39 mph at 5400 rpms, cruises at 25 mph inshore, burning about 7.5-8.0 gph at cruise.
My '71 Seafari with original Mercruiser 140 ran about 24 mph at cruise for 5 minutes before blowing the headgasket. No idea how much fuel I used.
THOSE are My claims.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury