I think if you're a passenger in someone else's car, you can politely express your concern over the GPS route once. If the driver doesn't take your point, then let them do the driving using map, GPS, written directions, or whatever navigation aids they choose to use. I'd make an exception if the GPS is sending you down truly odd roads--unpaved with grass growing between the tracks, up a mountain or into a lake, or into an area that you believe is dangerous--then speak up in the interest of safety, like the copilot who asked the captain, "Excuse me, but are you planning to land on that highway? And if so, are you going to lower the landing gear?"
I take GPS auto navigation with a grain (no, a full saltshaker!) of salt. My first experience with my brand-new GPS was when it directed me to a rural location on Rattlesnake Gutter Road (yes, that's really the name!) during a New England winter. Unfortunately, while the GPS plotted the location correctly, it failed to understand that 10 miles of the road between me & my destination (the part that goes over the mountain) was closed for the winter and blocked with 4 feet of snow. In addition, my GPS refuses to believe that I live where I live (probably because I live in a converted carriage barn), and every time I turn into my own driveway, Miss Prissy Voice informs me, "You are in the wrong location. Turn around when possible!" I'd sooner believe my horse--you can drop the reins and he'll always get you back to the barn.