Playpennies

hi i would say your dashboard ...but if your caught speeding it will be what the police/speed camera say you are doing anyway ...but your speedo should be the more acurate out of dashboard and gps as you shouldn't be relying on your gps for your speed anyway.

GPS is far more accurate than the speedo. Speedos generally read over, which makes the 64mph sound about right that you were travelling 60mph. Speedos have lots of variables as they're measuring rotating speed and the tyre size/pressure etc all contribute to inaccuracy.
Depending on your GPS, you can be pretty sure of it being as accurate as you'd need it (a cheaper gps may not account for the fact you're going up or downhill which would make your horizontal speed faster or slower)