I agree with what a lot of PPs have said: if you're the passenger, then it's like being a guest. Offer help once or twice maybe, but if it's not accepted, then you just have to sit and bear it. It's like if you're a guest at someone else's house and they're cooking. Even if you're a Michelin star chef, you still don't presume to tell them what to do beyond generally offering to help. It's not about what's best or most efficient. It's just the polite thing to do. And if you're a passenger in someone else's car, then you're a guest, after all.
Of course, I fully agree that a local will often know faster routes than a GPS, and different GPSs with different settings have varying efficiencies. It's never a sure thing, though, and it's the driver's prerogative to do what they feel most comfortable with. But after you initially offered to direct your friend along a faster route because you know the area well, I'd just say nothing unless specifically asked for advice. Things like, "I usually turn left here," aren't offers for help, and could be misinterpreted as passive-aggressive or nagging depending on the tone (I'm sure you were fine OP, but I had a coworker who would constantly say, "Oh, you're doing X? I always do Y," in a very superior tone about every tiny thing).