I have those wheels. Before on my Devonports (17") my Speedo and GPS were different. With 19" and 45 profile tyres both speeds are now the same.
that shud be about right $immo
std speedos are always too fast by around 10%
255 45 19 and 275 40 19 is pretty much about the limit for bigger than std tyres
im going for 275/40/19 again in the summer when i take my 17inch winters off