Be careful before you rely on your GPS to check your speedometer.
Depending on the make and model of GPS, you may be getting distance as
'the crow flies', NOT actual road distance.  Not a problem if traveling
on a straight road for the testing, but a problem if any curves in the
road.  Set a course into your GPS, then start down the highway in any
direction.  The 'distance and bearing" to the next waypoint will be
shown as if you could drive a straight line to the waypoint.  Most of
the earlier handheld GPS receivers were developed with boaters in mind,
and make the assumption that you are going in a straight line.  More
recent models are designed with the hiker in mind, and some are set up
specifically for use on the road.  Figure out how yours' is set up
before coming to any important conclusions.  The best bet is still the
measured 5 mile stretches on the highway, just take your stop watch,
cruise at 60 to make the calculations easy, time the 5 miles, note the
reading on the trip odometer at start and end, and back into a
correction.  I end up .2 of a mile long in 10 miles on the odometer, and
about 1 to 2 miles fast indicated speed on the speedometer on my '90
Westy running Michelin 185/14 Agilis.