You want to use Apple maps and not even launch the Google maps app. Google maps is a data pig compared to Apple maps. I just spent 10 days in Italy using my iPhone as a GPS, trust me on this.
Apple maps also allows you to pre-load a map before you arrive at your destination; do this on Wi-Fi the night before, don't close the Apple map, and you will save a lot of data upon your first launch.
BJ