I am thinking about going from my P215/65/15's up to a plus one, 225/60/16. I know the LTZ Lumina used this size tire, so there is a speedometer gear available for this combo. Question is, how easy or hard is it to do this? The tire calculator says when im doing 58 my speedo will say 60.

It's almost as easy to correct the speedo in the PCM.

It's almost as easy to correct the speedo in the PCM.
I'm not really familiar with how to do that and I'm sure it requires purchasing a hand held tuner, if not then I'll learn how. But buying a gear is usually the correct way, no?

from what i've heard, it's impossible to do without having a custom PROM burnt for the ECM