IAS calibration / correction ?

I read a lot about guys flying speeds faster than I see in my 0320/ RV3. I thought, what is wrong here. So I did two runs, four cardinal directions averaged, two different days, and the error is 19 MPM both times. I appreciate suggestions where to start investigating, correcting this disparity.
Relevant facts:
Standard Vans pitot
Flush static ports at Vans suggested location
Plastic tube ferrules and nuts on connections

I assume you know that averaging gps ground speed over 4 directions does not give you TAS - but unless the winds were really strong it shouldn?t be off by 19 mph. And you correctly changed IAS to TAS. I?d start with static port placement. Go to an airport with a tower. Look up its height. Set the altimeter to field elevation while on the ground. Do a low pass at 100 knots or so, at exactly the tower height. Does the altimeter read the altitude correctly?
Search ?water manometer? to do a simple calibration of your ASI on the ground.

Not sure about the 3. My 7 was out about 8 kts indicated. I had aftermarket flush static ports. After reading much info on the forum I changed the ports to the Vans rivet, which is domed (it is a baffle rivet), and my IAS was right on after that. Vans now sells a static port with the same profile as the rivet but has a better end for connecting the static tube. FWIW.

GPS =TAS ahhhh

Thanks guys. I am just a low and slow tube and rag convert. This high tech stuff is new to me. I will do it again capturing TAS, which is a PITA with minimal steam gauges.

Record indicated airspeed, pressure altitude (set alt to 29.92) and temperature. With that info convert IAS to TAS.

To see if you?re close, a rough approx is 2% per thousand feet. e.g., if you were at 4000? the correction is 8%. If you had 140 mph indicated, the true airspeed would be 140+ (140*.08)=140+11.2=151. Mph. But this is very approximate, look up the formula.

IMHO the best way to find out your real TAS is by doing the three leg GPS test. You can even use a handheld GPS for the test. There have been many threads about this exact situation here but here is a link to one of the threads.

The three or four leg GPS method is very reliable.

Here is a link to a page on the National Test Pilot School

https://www.ntps.edu/information/downloads.html

that has links to a discussion of the process and a spread sheet to use for the calculations.

I used it for both of my airplanes and periodically redo the tests to check that nothing has changed.

Just collect the data and feed it into the spreadsheet.