I drove from NYC to Chicago and back using the GPS on my 2012 (awesome!). I did notice that 15 minutes of real time was a few minutes faster than 15 minutes of GPS time (and I was doing about 10 miles over the regular speed limit). On a 14 hour trip, the GPS was about 2 hours slower.
I also noticed that the time on my clock changed automatically when I entered the next time zone. Cool!