As I wind-down my Phase I testing, I'm tweaking all the little glitches that I've noticed. Yesterday, I noticed my GPS ground speed was several knots better than my IAS (the EFIS reports IAS and TAS) - it was a calm day and the difference seemed to persist in any direction/track. That got me to wondering if my MGL Xtreme EFIS was off a little on the factory calibration (or to be fair - perhaps something was "off" in my pitot-static system - it's the Zenith supplied probe.) On the MGL forum, another user reported a 7% difference (too slow) with the same EFIS.

The MGL Xtreme EFIS can be recalibrated by the user. I stumbled across this Excel formula to derive both TAS and IAS from GPS GS. You basically fly 3 legs, record the GPS GS and tracks, and plug it in the formula. You can then recalibrate your EFIS to match the IAS values.

Obviously, this would be useful for any EFIS or airspeed indicator that allows the user to re-calibrate. Just thought others might like to have this formula: Calculate_tas_from_gps_gs_plus_AIS.xls