Yes, this is how consumer labeling works today. Net weights, cash register scales, gasoline pumps, etc. Errors are only allowed if they're in the customers' favor.

That said, sunscreen is hard to apply precisely. One interesting emerging option is personal "makeup mirrors" that use a UV camera.

Cars show higher speeds especially when the model has an option for larger tire diameters, but is equipped with smaller ones. There typically isn't a setting for tire diameter, so they compute speed using the larger diameter in all cases.

5-10%, definitely not. Wrong tire size will do that though.

Have had a GPS speedo on the dash for a good dozen cars through the years and never seen more than a few mph off on a flat surface. That's something I actually noticed and looked for, for some reason. A few mph over speed is fairly common, but we're talking 1-2% at most. (confirmed with Tesla Model 3, Corolla, Fusion, Prius, Elantra, Mirage, etc etc).

I know it sounds like a lot but in my experience the difference is mostly a fixed offset plus a tiny percentage due to tire pressure/size.

A fixed 2mph difference at 20mph is 10% so imho they're at least _technically correct_.

5% at 70 mph is 3.5 mph

10% at 30 mph is 3 mph

I saw this with various European cars.

My experience is that it seems to be a fixed percentage rather than a fixed amount i.e. the absolute difference increases with speed.