The heat from your engine can raise the ambient temp, mine raises about 6 degrees or more if I'm sitting in traffic on a hot day. My car is black too so it's usually a couple degrees higher than the temp on the bank sign I drive by, and will be higher after sitting parked in the sun all day even in winter. After driving down the road and letting fresh air hit the sensor it levels off though.

But for yours to read that much higher is a bad sensor

Oh and on my gp I added the mirror with temp and mounted the sensor on the bumper bar, still did the thing.