First thing I would do is take it to someone who really knows whether your Mooney is in rig and flying straight.
After that I would blame some cruise speed loss on your three blade prop - 3 to 5 knots. Then figure out if you want to give up the looks and the climb performance of the 3 blade prop for a better cruise 2 blade prop.
Finally it's your expectation - when your airplane was being built there was a lot of competition in General Aviation: Cessna, Piper, Beechcraft, Grumman, etc, etc. were all competing for sales. Very few airplanes that ever left any of these factories did what the POH said they would do. Expecting POH numbers for airplanes built in the 60's-80's leads to some disappointment. (Example: How many 201's did 201mph? Same goes for the 231 and 252? It was kind of understood that those were marketing numbers not what you were going to actually see.)