Not sure on the mileage question, but your GPS is not reliant on WiFi. My truck has the GM navigation and I think it sucks balls. I’d much rather use google or Apple maps. More known addresses and faster recognition in my experience. Also, I purchased an update for my factory Nav a year after I bought it and it was the biggest waste of money I’ve ever spent. Unless there have been HUGE improvements made to it since then, I will never give GM another dime for their crappy Nav system. Sent from my iPhone using Tapatalk