Tires and the Prius

The meters haven't been mechanical for quite some time.
There is an electronic speed sensor counting revolutions of a wheel or some gear in the drive train. The computer then calculates miles, then applies the pertinent adjustment factor before sending it to the corresponding display.

 

Go directly test this with your GPS.

Yes, the sensor is shared. But computers can 'adjust' the individual displays to fit the automaker's 'customer expectation management' goals, or legal needs. E.g. this is how a past car's coolant temperature display showed a rock solid, never budging temperature. After getting a ScanGauge-II, I discovered that the entire temperature range from 145F to over 210F, was all condensed to a single point. Temperature changes within that range were undetectable on the dashboard display.

Speedometer offsets are a very long standing industry practice. Long ago, we even had numerous pointers in these forums to UN-based Euro-market standards showing and regulating this. North America adopted different SAE-based standards. Now that the mechanical meter age is gone and we have much more consistent digital meters, the makers can simultaneously meet both standards.

With a GPS, you don't need any citation, because you can go directly measure it for yourself. Also get an OBDII-port engine monitor (or Bluetooth dongle paired with a mobile app) to see what the car's ECUs report internally, separate from what the dashboard meters display.

On my former Gen3, the dashboard speedometer read 1 mph higher than what the ScanGauge-II reported from the CANBUS / OBDII port. Some else dug into the matter, and found that the difference is added in right at the speedometer display circuitry.

Both my own GPS, and several very long series of highway mileposts, determined that the odometer read a bit over 1% low, the opposite direction from the speed display 'error'. Highway mileposts are a bit tricky, because though they are set (in my state) according to the original highway survey, later road re-alignments can cause discontinuities or breaks. A few posts are also offset for installation convenience, where the correct location is a bad place to install a post. But over long distances, the offset posts average out, and the breaks are detectable and you can still use the road segments between these discontinuities. Out in the plains (away from deep canyons), the GPS doesn't have these problems.

 

Are you really that concerned with a slight difference in readings?
FFS

 

Yes, Discount Tire allows you to do that. It's not my first set I've returned. Plus some tire manufacturers have a generous return policy, usually 30 days but I've seen some as high as 60 days.