Entering the "hidden" screens while driving appears to reveal the LEAF's GPS-calculated speed.
NOTE: This is most safely done by a passenger, NOT the driver (who should be looking at the road).
Enter Hidden Screens:
1. With the Audio system OFF, press the MAP button 3 times,
2. Press the Audio System Power button twice,
3. Press the MAP button once again.
You will see two choices. Choosing the bottom one ("Confirmation/Adjustment"), then "Navigation", then "Sensor Information", and some of the GPS data will be displayed, including "Vgps", which appears to be tenths of kilometers per hour.
A reading of 1000 would be 100.0 kph, which is 62.5 mph. Your speedometer will probably "lie" and indicate something like 1.5 miles faster, or about 64 mph.