I have a 1990 Sunbird corsair 205, OMC 5.0 Ford with 2bbl carb. I had a 19 pitch prop and am trying a 21 pitch now. At full throttle, my tach says 3000rpm, speed is 42. This is with a full tank of gas and 3.5 people on board. I have hit 45 max with a lighter load. I don't think my tach is...