I had to see for myself. I left the house at 10:39. I fired up the Pi without the modem connected and drove to an 11:00 appointment. I left that appointment at 12:22. I plugged the modem back in and used CarPlay, etc. on the drive to lunch.
Teslafi shows me being parked for 1 hour 42 minutes. It did notice some range loss during that time and noted "A drive may have occurred during this idle session. The range loss and kWh loss is not included in the daily summary."
So for that reason alone I don't ever want to use my system offline to save a few bucks on a data plan. I'd like to find a way to do it for less than the $25 per month I'm paying now.
Also concerned about what else I might be giving up. I've noticed that remote access to the cameras don't work as well when the Pi is on.
Wonder about emergency services? Apparently the Tesla will contact an emergency dispatch automatically. Does it know how to do that if it's connected to WiFi?
Learn about the automatic and manual emergency call system.
www.tesla.com