2 deg. C too high

I just replaced an old probe and receiver for a Digital Davtron 301c.
My old one started reading cabin temp and I simply replaced it
altogether (probe and receiver). My new one seems to read 2 deg. C too
high. Any ideas? The probe is hard-wired from the factory (so, the
probe is correct). The probe is mounted through the sidewall near the
pilot fresh-air intake. Oh by the way, the plane is fiberglass:
Glasair II. I have compared the reading to other planes on the field
and mine is consistently 2 deg. high. Hard to find anyone at my
altitude to compare in-flight.

Also, I have only been flying for about 4 years, 3 instrument rated but
have had an idea: doesn't is seem that it would be more advantagous,
for atc, to use calculated ground speed instead of TAS when filing?
Expecially in this age of not-complete but prolific radar coverage and
pretty accurate winds aloft models? Just a thought.