Having problems finding an accurate GPS speedo lately.
Our two modern cars have Australian ones that are very good, but no longer seem to be available. These ones have been checked by radar and are spot on. They were advertised as being accurate to +-1kph if I remember right.
They have also previously been recommended by a US hotrod magazine.
Having bought another vehicle I bought a cheaper Chinese one and it is wildly inaccurate, so it was sent back. On doing some reading the only accurate one seems to be this
https://www.ebay.com.au/itm/US-HP02-....c101195.m1851
Now, these have their own battery which may make them good in a vintage car, or needs to be portable, or/and when you don't trust that old speedo.