The key is to have evidence that your right.. If you had started out proving yourself, she'd be trained to accept your answer as right by now. My girlfriend knows that I'm right ~98% of the time. You do have to admit when you're wrong to maintain your credibility though..
One time we were on a road trip and we debated on which side of the highway this town's Walmart would be on. She was right and I hear about it just about every time we go to Walmart..