the 3.7 is best to turn in teh 4500-4600 wot rpm range, they will turn higher, but it tends to get tough on valve train. there was a guy in Atlanta running one of those engines in a dragster, I think he was turning it over 10, but he had roller rockers and changed from a chain drive cam to a gear driven($$$$$$$). 5000 isn't going to break you bottom end, but I doubt you are making good power at that rpm, you're above the curve with the stock cam/carb/ intake/exhaust package. The 3.0L drive came with a 1.98 ratio adn the 3.7 with a 1.82 ratio. The 1.98 was the alternative ratio for the 3.7 in high altitude set ups. First thing you need to do is check your tack against a shop tach, usually only about one in ten is even close to being accurate, next borrow a gps(hand held will work fine), check your existing set up. I doubt your are crusiing at 40 mph, that would probably be closer to the top end speed. What prop are you spinning? I'd try a 19 or 21 Mirage
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