I have the 3.55, and added 33x12.50 on my stock17" wheels and i'm told it would throw my speedomoter off by 3 or 4 mph, but my rpm is exactly where it was before at 70mph, am i missing something here?

 

Check out this site.
www.miata.net/garge/tirecalc.html
Put in your stock tire size and new tire size. It will tell you how far you are off. Going to 33's, your probably off by 10-15%

 

Yep your off, you changed your final drive line ratio with the bigger tires. You can offset that by chaning the gears or by having the spedo computer adjusted (a whole lot cheaper). The speed diffrence is a percent diffrence as Scott mentioned below.

So if your spedo says 5 MPH you may be going 6, if your spedo says 70 you may be goin 78-80MPH. So watch your speed and call your dealer and ask about having your spedometer calibrated for the larger tires, they should be able to do this with out much of a hassle.