I am aware of speed detection via gps, however as a background app, I am concerned about high power drain. I am looking to detect speed, say over 5mph, but it does not have to be accurate, say like a speedometer.

Is there a low power way to detect if the phone is in motion, say by triangulation, or tracking tower strength and new/recently lost towers?

I have an app design that is dependent on running in the background upon launch and knowing if the phone is in a car or not.

Thanks!