Let it run!

I have a 2012 Camry with remote start(factory junk) and a 2010 Prius.No remote start.
We used to take the Camry to do our running when it was -10 to 30 and use the remote start (if you were within 50 foot) Then I discovered I could take the Prius and set the heat in the low 60's and leave it running. Of course you have to manually lock the doors. As is its nature the engine cycles and over an hours time my mileage shows a drop of 2/10's plus the car is toasty warm. It actually uses less gas than the Camry when remote started. Plus the Camry would have to run 10 min. to get that warm inside.
We don't always do this but,it does work well when we do.