If you're doing it on an EF/L you can use the Dash Diagnostic mode to find out exactly how fact you're going:
* Turn Smartlock Off, put your keys in the ignition and hold down the odo button. Count to about 12 (keep holding it in).
*Whilst still holding the button in, turn the keys to the 'on' position, but dont start the car. Let go of the button, then push it quickly 3 times, holding it in on the third time. Hold it in until the seatbelt light (in the middle of your teacho) goes out, once it goes out let the button go.
*If you've done it right, the odo should black out for about 5-8 seconds, then start flashing 'FORD' at you. You can now turn the car on.
*Once you're driving push the odo button until you see a '3' in the top left hand corner. At this screen, the bottom row of numbers is the vehicle speed, and it is exact. If your speedo is set right it'll be 3kms out (this is normal factory setting), so 100 on your speedo will equal 97 in your odo doing this test. The top line is engine RPM.
*If you keep cycling through (by pressing the odo button to move to the next screen) you can find battery voltage, fuel tank level etc etc - lots of other things that I cant be bothered writing out. If anyone wants to know the rest I'll post it up later.
*To exit dash diagnostic mode, hold your odo button in for about 3-5 seconds. It should go back to normal after that.
WARNING: If you're driving with Dash Diagnostic mode operating, it is more than likely your engine will stop for half a second or so after around 10 mins of operating in this mode. Your gauges will go back to zero, then they should return to a reading of your current speed/RPM, and the dash will automatically exit diagnostic mode.