Instrument Cluster

I'm looking at a 99 Cobra and the speedometer is inaccurate. The car didn't feel like it had a rear gear swap. I was wondering what might cause this? Also, if I change the instrument cluster is the odometer reading recorded in the instrument cluster or some kind of brain box? I would like to switch to an 03/04 cobra cluster will this change the odometer reading? Any info please. Sorry I've never owned a sn95 car so its all new to me.

Any help?

They are stock size tires. lets say they don't feel 3.55 or 373 because I've had so many cars with gears and the rpms just aren't there. What else can cause it to be incorrect? "Where is the rpm recorded is it in the instrument cluster itself?