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Question about Air Speed Indicators and accuracy.

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This year at annual, there were issues with the paperwork supporting the engine change done in 1971.. So I purchased the Bold Warrior STC to cover that concern. As part of the STC, a new ASI needed to be installed, along with the fact that my original gage failed the Pitot Static check. The gage was leaking. Installed a new United 8100-B.154 gage with the proper markings. Had the system again tested and certified. All was well I have thought.

As part of the annual, the Laminal Flow Fancy Pants wheel fairings were also installed. This was the only change other than the engine airbox and ASI per the engine STC. Wight and Balance was also measured, and yes, the plane did gain weight. 1392 empty with 6 qt of oil.

Since getting the plane back up and flying, have noted the ASI shows 10 to 15 MPH Lower speed... GPS shows the same 150 to 155 MPH that was before the annual. I have done stalls before with the old ASI and noted that full flaps, stall was at 58 to 60 indicated. GPS was about the same within a few MOH.

Now, with no other changes to the wing, stall with full flaps, at 1900 lbs flying weight, is showing less than 45 mph., the light comes on just about 45, and the stall most likely about 43.... the GPS is the same as before... 58 to 60....

So, the Pitot system was tested and certified. I was not there so do not know what the process is or how it was calibrated. What are some ways to verify that the system is correct, and that the indications are right???.... Not sure the shop that did the certification wants to come back again, as they had to make 3 trips out to test it before.... What else can be done.???

The plane feels like it is going 10 mph faster at landing also.... normally I fly 80 on final and 75 over the numbers... at that speed it floats some before the landing... and never have gotten a stall light during landings...
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