I used Maps on Sunday for a short 20 mile direction trip. What I did was set up the Start and Finish trip using Wi_Fi from home this seems to cache the trip so using no data, I then clicked on Start when leaving in the car. I had Cellular data tuned off in the Settings and left Wi-Fi on. This worked well and was using voice navigation. I also have Wi-Fi Assist turned off in Settings.
There's varied opinions I've read on data usage with Maps when using for voice navigation, most claiming it doesn't use lots of data? Probably the best way to find out is use it for a short trip in your area, that should give you a good indication of any data usage.
This is a great recommendation Salisbury. Data usage with map application will vary based on the length of the trip. Using it on a short trip will give you an idea of the amount of usage. I personally (with Google Maps) would map out where I am going and use the written directions offline versus the turn by turn to prevent high data usage on long trips. You can do that as well Rackem.
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