Yes, it's mph. If it was using imperial units, it would be kph.
I don't know what happened either. One minute the top speed was ~115 (I just HAD to test that 118 mph top speed

), and the next it was at 177. It always stays in its little holder so no I didn't toss it anywhere while going 118. Futhermore, I'd have to throw it around 50mph forwards and as I'm already in the front seat, it would be slightly bad for its health (not to mention whatever it hit).
I'm sure for the most part, GPS is accurate, but I thought this was interesting. Makes you wonder now if all measurements are totally accurate. Hmmm.