Another simple idea:
First message (after reboot of device or after reconnect to gateway after a break) contains complete Lat Lon location following one of the scheme above.
Following messages contains offsets only. For a 40KM range that is about 4bytes (2 for lat +2 for long).
Practically, no vehicle (and device) can travel more than 15KM in 10 min at 90kmph. This is much less in urban area due to traffic.
Venkat.