So you thought your gauges were precision instruments? Well let me awaken you, rudely! Thank goodness the PCM controls combustion a little better than this.
Various 2011- 2015 Vehicles - Cruise Control Speed Setting Display May Not Match Speedometer
Some 2013-2015 Fusion, Taurus, 2011-2015 Explorer, 2013-2014 Edge, Flex, 2013-2015 Lincoln MKS, MKX and 2013-2014 Lincoln MKT, vehicles may exhibit a cruise control
speed that does not match the speedometer speed. The Instrument Panel Cluster (I PC) provides a tolerance which allows the speedometer gauge to display between 3% lower and 7% higher than the actual vehicle speed. This means that with an actual vehicle speed of 96 kmh (60 mph), the speedometer may indicate between 93 kmh (58 mph) and 103 kmh (64 mph), which is normal. Refer to Workshop Manual (WSM), Section 413-01 Description and Operation, Instrument Panel Cluster (IPC). If you are outside of this tolerance range, refer to the WSM for normal diagnosis.