"I guess you could always cap off the exponential interval increases at some max interval, making it basically a repeating timer."

No you MUST have a max interval cap. Exponentials and all, you'll soon be waiting days or months for a retry.

It's ok, if you wait long enough it'll go back down to a small number. Or maybe a negative number and then the car crashes?

Please cap your numbers for both user experience reasons, and safety reasons.

Yes, have dealt with this before. Nothing like doing a fault analysis and telling someone that it'll fix itself in a few months.

A capped fibonacci sequence is a very common back off timer.

Most (actually almost all) IoT systems are not well-thought-out in this regard. It requires a bit of paranoiac foresight. Designers far too often think it's really, really important that remote devices stay connected to the mothership, when in reality, most IoT devices would be far better off NEVER requiring a cloud connection, but leveraging one if/when available.

*Rule One of IoT network design: NEVER assume that you even have a network * If at all possible, and to the maximum degree possible, an IoT device must work whether there's a network available or not.