Waze is an app that you can download from Apple or the Play Store, it is free, more accurate than most paid for SatNav software, and if you don't mind Google knowing your whereabouts, the way to go if you don't have a built in Satnav in the car.
For 1 hour driving in 3D mode, it consumes up to 4MB and in 2D mode, it takes up to 2MB, so you can work out if your data allowance will cater for it.
As others have said, download your route at home before you set off to save on data usage. and do get a reliable phone mount for the dashboard or screen, the number of times mine fell off the screen at a critical moment was one of the drivers to move to a car with SatNav built in! 