looking for some instrument debugging help.

I have a steam gauge ASI (winter) and an LxNav S80 sharing the same Pitot and static lines. In the air, the two gauges read ASI differently. The S80 reads about 10 knots higher than the Winter at 70 knots, and 5 knots higher at 40 knots. The error seems to scale with higher airspeeds

Back on the ground we used a syringe to pressurize the pitot line. Both instruments read within 2 knots of each other for the full speed range. they also held steady at the desired airspeed with the syringe, indicating no leaks in the Pitot system.

Back in the air, I tried opening and closing the vent, scooping air into the cockpit to pressurize it, etc. Neither instrument changed its reading, but the offset in speeds remained.

Ideas? Leak in Static?

Matt