I have the Insight TAS-1000 in my airplane.

I've noticed that there's about a 5 degree C rise in air temp
when flying at the top of the green (165kts or so as memory serves)

One can count on getting ice when the TAT is exactly zero.

I've often wondered why the variation... the OAT thermometer measures
static, I think.

The reason the jets have little trouble with ice is that they go so
fast that
TAT remains above zero until the OAT is so low that ice is not a
problem.

Bill Hale