I know 911s have great heaters but the roof-on when snow is on the ground is a good general rule of thumb. 
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Glen
49 Year member of the Porsche Club of America
1985 911 Carrera; 2017 Macan
1986 El Camino with Fuel Injected 350 Crate Engine
My Motto: I will never be too old to have a happy childhood!
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