Newby question?(dont laugh)

Don't worry, offset has nothing to do to your speedo. What you need to worry is your tire size.
Here is the link that you need to calculate the right tire size that you need.
http://www.miata.net/garage/tirecalc.html

Newby question?(dont laugh)

I have a 2000 prism with 75000 miles on it not that that has anything to do with it but.....I want to put some 17s on my car but my question is what is the offset business that I here about. what is the difference? I want the ride to be ok, I am not expecting a great ride but a good ride, will it throw off my spedometer at all and will it make my suspension be shot after I take them off for the winter time. Also what type of offset would be ok. realistically how much slower will my car be off the start and anytime inbetween. I know thats a lot but any advice would be greatly appreciated.

Want to know about offsets then go click Here