my corsa b had 15"s on and there was 5mph discrepancy comapred to my sat nav, think the sat nav was like 5mph slower than what my dashboard was reading, but there was several times when i was followed by a cop car and went through loads of speed camera's at 30 (according to the corsa speedo) and didnt get pulled or flashed,
now in the clio, got the same wheels on, the original on the clio were 14"s, and its only a 2mph discrepancy
p.s. will go out and check now what my tyre size is as im off to the burger van lol