They aren't that different from each other, and they are all at room temperature. Perhaps, they are worse at -5°! I would like them to be within 1°; maybe that's asking for too much. And, the electronic ones on display are not powered up.

Day time temperatures are measured in the shade. Make sure your Taylor dial thermostat is not being hit with direct sunlight. If the thermostat is right up against the house, then any heat loss form the house could also effect the temperature reading.

It's mounted on the post which is supporting the roof of our east-facing porch, 10 feet from the house. No sun, ever!

Right, Elmer. The internet's weather prediction for my town for today is a high of 7° and a low of -15°. My porch thermometer says +5° and my car said -2° at 10AM today.

COLD!

Try temporarily mounting the Taylor thermometer as close to the car temperature sensor as practical. It should only take a few minutes to see if the variation persists.

Car temperature sensors are often at or near the underside of the car and readings in extreme conditions can be affected by the temperature of the roadway.

If you look at the cheap dial thermometers lined up on the store shelf you will note that they all indicate a slightly different temperature, a few much father off. Pick one that reads the median temp of the bunch and it will likely be close.

I've got three different remote sensors from Ambient Weather and they are consistently within a half degree of each other despite each being about 100 ft away from the others. (I had one in the barn and one in the house, then added a weather station. Haven't bothered to line them up next to each other as I don't need that degree of accuracy.

Mostly I look out the window to see how tightly rolled up the rhododendron leaves are. When they make really tight little cigars I put my long johns on.