Dynon TAS vs GPS TAS

[Cam]

I've been trying to check my pitot-static system on my RV.  I have a Dynon heated pitot tube in the Vans location.  The Dynon OAT is under the HS (it's checks out good with two other temp gauges).  There is a Vans ASI on the same pitot-static system as the Dynon.  I've been using a GPS and making 4 way runs to find the TAS from the GPS data.  

So my question is, should the GPS TAS be the same as the Dynon TAS?  I've found that the GPS TAS numbers are always higher than the Dynon TAS numbers.  Here are some numbers I've been getting.  Speeds are in MPH.

ASI IAS   Dynon IAS   Dynon TAS   GPS TAS
 82             74               77               94
 104           99               105             119
 160           158             168             182

If the two TAS are suppose to be the same, any ideas on how to correct the pitot static system to reflect the correct speeds?  I did a pitot system check before hooking up the pitot tube using a manometer and the Dynon checked out lower than the ASI from Vans, about the same as what was observed from the flight tests.

Thanks for any help understanding all of this
Cam

 

[dynonsupport]

Cam,
We do expect that the EFIS will match your ASI in IAS and will match the GPS in TAS. What you are seeing isn't really expected and sounds like a possible problem with your unit.

If you could, please send us the serial number and model of your unit in a private message by following this link:
http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?action=imsend;to=dynonsupport

We can use this information to look at the history of your unit and see if anything looks out of order.

We may need to get your unit back for repair, but we will get it fixed for you.