79 johnson 70 vs 73 evinrude 65

i have recently changed my boat motor on my 18 foot amf crestliner from a 1973 65 hp evinrude 13-3/4 x 17 prop to a 1979 70 hp johnson 13-3/4 x 15 prop... the old 65 would push us around 29 mph and we can only get about 25 mpg out of the 70 johnson...sounds to me like its not running enough rpm we dont have a tach...i cant imagine the 65 out doing a newer 70....its funny the old 65 has a more pitch prop than the 70..and i know the 65 ran more rpm than this 70 does...i know im getting full throttle out of the 70 and it idles and runs great..any help would be appreciated.......also another thing is the old 65 had a small maybe 14 inch stabilizer fin on it and this 70 has a larger one on it maybe 18 or 20 .. i dont know if that makes a difference or not.

Re: 79 johnson 70 vs 73 evinrude 65

Your descriptions contradict each other, stude. You need a tachometer. Opinions won't get your questions answered. You don't know what either engine turned in rpm, you are going by ear, not instrumentation.

Re: 79 johnson 70 vs 73 evinrude 65

ill get a tach...how much does a poor prop effect performance...could it not let the engine get up to full rpm because of wear or chips..the reason i ask is i dont want to buy another prop only to find out later that i need a different diameter/pitch prop to get full power for our boat set up because like i said the motor was just put on the boat. i would just like to here a few ideas to check before dumping money into an unknown problem.