One more item that I should have mentioned in my first message:

Airspeed system accuracy - you say you are 10 kt slower than the
manufacturer's claims. It isn't clear how you determined the airspeed you
are getting. Airspeed indicators have errors, and the instrument error is
often several kt, and sometimes many kt (if you have an unserviceable
ASI). You almost certainly also have some static source position error,
and this can add another several kt error in the indicated airspeed.

If you have determined TAS using the average of GPS runs, be aware that
some of the methods that people propose are not mathematically sound.

There is lots more info on how to calibrate ASIs, determine static source
position error, and determine TAS from GPS data in the Flight Test Links
section of my web site:

http://go.phpwebhosting.com/~khorton...dex.php?&PID=1

So, depending on how you determined your speed, maybe you are 10 kt slow,
and maybe you aren't.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com