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How much data do you use on your mobile using google maps as a sat nav?
Not much.
GPS doesn't use data at all. "Assisted" GPS uses a tiny amount of data to speed up the initial acquisition of a satellite lock. This is true of all GPS systems, not just Google.
Google Maps needs a data signal to load its maps. You can minimise this by planning routes before you leave the house (ie, on Wi-Fi) and it will cache most of the data it needs. Traffic updates also need data, but that's miniscule.
The last time I went on holiday I did a US road trip across three states (and back), Google Maps was my sole navigation and data usage for the two weeks was in the order of megabytes.
Download maps and it'll have them offline.
You can download some huge areas, the entire south east of eangland from bath eastwards, below central london, is 370MB.
1.5gb of mobile data since 8th January apparently. I’d imagine some of that was downloading my offline maps that live ok my phone.