It looks TCH meets following "ECE Regulation No. 39".
http://www.unece.org/trans/main/wp29...egs/39rv1e.pdf

On the last page, it says...
0 < or = display error < or = (real speed)*0.1 + 6km/h

The Japanese Prius displays about 3km/h higher than real speed when the vehicle speed is 40km/h or higher.

Ken@Japan