To properly determine instrument error, you need to send your ASI to a shop that can calibrate it. You will get a table with calibrated airspeed versus indicated airpspeed (and an invoice!). The difference is instrument error. Only then can you say that your Van's airspeed (or one from another manufacturer), is off by XX knots. Once that is determined, you can proceed to find position, or static source error, using one of many good methods. Calibrated airspeed = indicated airspeed corrected for instrument error and position error.
Mine too. My aircraft uses a Vans ASI as backup to my Dynon D120. I have 2 pitot tubes. One is the standard vans tube plumbed to the Back up ASI, and the other is the Dynon AOA, plumbed to the D120. Both instruments share the same static source. The Dynon reads within 2 MPH of calm air GPS ground speed, but the steamer is 7MPH faster at low speed, and about 5 MPH faster in cruise. One of these days I am going to swap pitot lines to determine if it is the instrument or the pitot input. Anyone else used 2 pitots??
Regards,
Chris