The rules I'm aware of are of that affect this for our cars are not the US ones but the European UNECE rules. Apparently there are punitive measures in place for the manufacturer if they make a car that goes faster than the speedo indicates (leading to unintentional speeding). This causes them to err on the side of caution, with around 3 mph being common for the Porsches and BMWs I've owned (my Passport Max tells me the true speed, it has always been spot one with any roadside speed displays when those have been set up).

The detailed rules are here: http://www.unece.org/fileadmin/DAM/t...gs/r039r1e.pdf

But a good summary is here: http://en.wikipedia.org/wiki/Speedom...nal_agreements

 

It makes sense to me that initially, on new tires, the speedometer is, almost without exception, a little optimistic. Later, as the tires wear down to the their lower limits, the speedo should read even more optimistically (since now the wheels are rotating faster than ever before). That way, the driver can safely (that is, without incurring a speeding ticket) "speed" on over to the dealer for a new set of tires.
https://rennlist.com/forums/graemlins/banghead.gif

 

On my trip out west I noticed a 2-3 MPH difference with WAZE and google maps. Car speed always lower than indicated on speedo!
Boston to. Santa FE so far, much more to go.