Ever since I got my GPS I've noticed that many of the cars I've driven have speedometer error. Usually they read a bit faster than the actual speed. I think I heard somewhere that there is some rule about it, that they can read a bit faster but shouldn't read slower.

So I wonder if that Scion TC has error, it may be reading faster. I want to say that the smaller the wheel diameter, the greater the error is at higher speeds.... but that could be wrong.

I prefer to use GPS over anything else when testing speedo error.... or those radar display things that the po-po likes to put up every once in a while.

 

Smaller tires change the reading the same as different rear gears do. Also keep in mind the speed difference they cause is a "ratio".
A 5 mph gain at expressway speed equates to way less than 5 at low speed driving.