I had this situation recently, and though it is over and finished, it's happened a couple of times with this same friend, and I'm wondering if there is a better way to handle it in the future.
A friend and I travel together for a hobby. We each have GPS systems, but they are different brands. The systems often tell different ways to get to a place, just as Googlemaps and Mapquest might differ; there is often more than one path you can follow to the same destination. A lot of times, the GPS (any GPS) or map website doesn't tell the most efficient path to follow, like a local would, but it's OK - give yourselves enough time, relax, and you'll get there eventually, is how I normally feel about that.
Recently, my friend and I were at a location I've been to many times, a few hours from my home. We had to travel back to my house, and we had time constraints. We were in her car, using her GPS, and it was telling us to go a different way. I knew that the way her machine was sending was perhaps technically less distance, but it was winding back roads with stop lights and would take longer than the route that took us a little out of our way, but quickly got to a main highway. My friend paradoxically wasn't sure about following her own GPS, because she recognized that it was sending us a different route than we had followed in the past, but every time I said something like, "I normally turn right here," or "We can get to the highway quickly if we just hop left here," she would dismiss it. We ended up taking a very circuitous route and it was a pretty frustrating drive which added an hour to a 2 hour trip.
Is there a polite way for me to disagree with her GPS, and to assert that I do have a good idea of where to go? Or am I stuck just letting my friend, the driver, follow what I know is incorrect? This wasn't my own local neighborhood, where I would clearly know all the best routes and shortcuts, but it is somewhere I have visited many times in the last 5 years, and I am quite familiar with the route, particularly compared to my friend.
And yes, one thing I will try is for us to NOT schedule time constraints into a trip like that again.