The first step is to figure out whether the indicated values are accurate. Tachs and IAS can easily have significant errors.
Check the tach for accuracy. You should be able to borrow an optical tachometer locally from another builder or an A&P.
Check the static and pitot system for leaks, as they can affect IAS accuracy.
Check the ASI for instrument error using a water manometer. You could easily have 10 mph error. You can get a spreadsheet I wrote to convert between water manometer height and ASI reading
here.
Conduct a static system error flight test. Static system position error could be more than 10 mph. You can read my notes on how to do this test and interpret the results
here, or you can find the articles I wrote in Kitplanes a year or more ago.