The odometer would stay the same unless you get it programed, which the company that does that is not expensive at all. and as long as i know it is a plug and play. I will post a link in about 20 min when im back on my laptop about swaping guages. There are three plugs on the back of the cluster and they should plug right in. i will look for you.
-----Added 11/4/2009 at 08:56:55-----
it took my about 10 min to install the STi cluster into my car.