I’m finding my ‘18 Prime Advanved speedometer reads approximately 3.5% high...and therefore the odometer as well.
Yep that is the way it is for all most all vehicles sold in the US. Mostly in the 3% to 6% range but can be 10% for exotic cars. Add a couple more % as your tires wear out. The only law is a Federal rule that commercial truck speedometers be + or - 5mph. Much discussed here on Priuschat.
As @JamesBurke kindly points out, this is by design, as has been discussed here many times; see my replies from October 2017 and earlier this month, for example. The speedometer and odometer both take input from the wheel speed sensors, but as far as I know, only the speedometer display is made to read high. I wouldn’t assume the odometer is inaccurate unless you’ve checked it on a dynamometer or over a measured course, or the Data List feature of a Toyota Techstream diagnostic system were also to report higher-than-actual vehicle speeds.
