On a recent round trip flight, I mounted a few extra thermocouples to see how far off the air vent location really is. I have a 360 with the primary OAT probe in the passenger side vent. I have been curious as to possible error, but didn't give it much thought until the recent discussion.
I mounted a comparison probe between the rear wing spar and the flap about a foot out from the fuselage.
The results in steady state cruise showed the vent location to be 4 degrees warmer. This was independent of altitude, as well as, vent position, open vs. closed. There was a noticeable lag in the temperature reading out on the wing during altitude changes. In climb the temperature delta would get smaller, down to one or two degrees. In decent, the lag would produce a larger delta, as high as seven degrees.
FWIW, your mileage may vary.
Chris Zavatson
N91CZ
360std