Yeh, the speedo will be off.

Compare the revolutions per mile/kilometre for your old tyres, with the revolutions per mile/kilometre for your new tyres (these can be downloaded from the manufacturers website).

Work out the percentage difference, and add that to the speedo's reading (carry a calculator until you the hang of it). IE: 60mph + 5% =63mph. after a while you remember the how much to add per 10mph's.
<img src="/forums/images/graemlins/cheers.gif" alt="" />

It's time for my pill now!

93, 3.0L LWB, Pajero. 200K klm,s.
91, Mazda Bravo