Or simply set your trip odometer to zero when you pass a mile marker and drive a few miles, then check the trip odometer against the mileposts. If the odometer is off, the speedometer will be off as well.
However, most speedometers read a bit high (65 when you're really going 63). Yours is reading low (11 miles when it should be 13). If it really is low by 1-2 miles on a 13 mile trip, then you are reading 8-15% low, meaning when you think you are going 65mph you might really be going 76!
The most common reason for this is that the wheels and/or tires are not the factory size. Otherwise, I would want to get the speedometer/odometer unit calibrated or replaced, at that level of error.