I notice that the speed on the speedometer does not match the one indicated on the GPS. The speedometer is 2 miles per hour faster than the GPS. Which one is correct?

 

A 5% error is acceptable by manufacturers standards, 2mph at 60mph is only 2 to 3 % error it is more than acceptable.
Trust the fastest if you dont want to get in trouble with the law inforcement people.

 

I noticed this but the other way. I have the NAV unit set to alert me when I hit 80. When it goes off I look at the speedo and it is always at 78. I tend to think the GPS is more correct as it is based on real data. That alert goes off too much, maybe I'll up it to 85...

 

Likely if the cars speed was measured with a laser you would see the laser showing a lower speed than shown on the speedometer.Some believe cars are set that way to try to keep you a few miles per hour under any limit.
But at any rate all of these measurements (car and gps) are slightly inaccurate due to error in measurement. If I was very worried about it I would trust the speedometer because it is likely to be set low.

 

All my car speedos read slightly low (conservtive) for good reason.