The basic checks are,
1. Do you have the Van's static ports? If no then your static reference will account for most of the error. It is the outside shape that is critical, doesn't matter how you hook up to the tubes inside. Verify by using the GPS calibration method.
2. Calibrate your tacho and fuel flow.
If you are really turning the Sensenich prop 2400 or 2500rpm and have all your fairings installed and straight then you should be achieving close to Van's posted speeds.