super_dn wrote:yes ride is more bumpy, kinda, and yes speedometer changes
Yes yes and no.
If you get the proper size tire you can match the factory ratio.
http://www.miata.net/garage/tirecalc.html
I guess, I could theoretically say yes it will be off, but if you get it close it will be between 1% and 2% faster or slower depending on what size you go with and you really wont notice the difference..
Use that calculator to get the right size for you. Also keep in mind to get the proper offset and make sure they don't rub.
Honestly, the ride with be very rough compared to stock (if there are a lot of pot holes around). If there is a lot of pot holes where you live, i'd go 19 or 18... Your sidewall is going to be minimal, if you hit a pothole at high speed, it won't be nice.
Before I do so I need to know some things. 1.Does your car ride differently?? 2. Are the brakes affected? 3. Do I have to recalibrate the speedometer?? Oh and one more thing. I need to get new brakes. Any suggestions other than the oem or buying a big brake kit?? 
This is my first major mod other than cf roof & deck spoiler and c-mod grill.