True. Here in North Carolina the ground temp STAYS about 55 degrees when you get 8 to 15 inches underground ALL YEAR ROUND. Some of the formulas above (they are correct for the numbers they use but they) are not real world. Ground temp does not daily change 20 degrees.
The ground temperature worldwide is 55 degrees in the inhabited areas. That holds true for at least several hundred feet down, as water from 500 foot wells is normally about 55 degrees.
About the saving money bit. I remember during the Carter induced energy crisis some ‘expert’ came on the radio in MN, in the middle of winter and said the best thing to do was to sart your car and immediately start driving, not to give it even a minute to warm the engine. I often wondered how many engines he was responsible for wrecking.