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...
