I'm not sure about the US but here those can be optimistic as well to make you slow down. A GPS device is in my opinion the best way to get an accurate reading.speedo is optimistic. Best to fly by one of those speed-limit signs with radar readout.
Based on the power available, I'd say that is very close to the actual top speed.(SNIP)
I think one of magazine testers used radar and pegged it at 87mph.