Personal Crisis in Western NC: HELP!!!

Ok, the results are in!!!

Speedometer/Phone (+/- 1 mph)
10/9
20/17
30/24
40/32
50/41
60/49
70/55
80/64
90/70
100/77

The car speedometer at 60 mph takes 1:13 between mile markers. 1 minute between mile markers is 74mph +/- 2 mph (I had a hard time being consistent and watching the road.