My air speed is off reads slow and off more at higher speeds 5kts at lower speeds and 10 to 15 knts at higher speeds all lines are ok using dynon and a mech gauge both new and give same readings also pitot tube looks ok
Static system position error - possibly caused by using something other than Van's pop rivet static ports. Static ports that are flatter than the recommended pop rivet will cause the sensed static pressure to be higher than it would have been with the correct static port. This would lead to too low an ASI reading, and a too low altimeter reading. Many people have reported this can cause errors in the range of 10 kt.
Pitot system leak.
Static system leak.
ASI instrument error. I've seen reports of ASIs that were found to have 10 kt instrument errors. Have it checked by an instrument shop, or do it yourself with a water manometer.
Bad test method, or calculation error. E.g., TAS is not equal to the average of GPS ground speeds taken on various headings. I like the NTPS four leg method.
OAT error, if the test method relied on OAT as part of the calculation. This would likely only result in calculation errors of a couple of knots, so it wouldn't be the only problem. But it could play a small part in the total problem.
