so right now i have 235/75/15 and if i remember correctly my truck came with 195 14's something like that they were little tires but i am thinking of getting some m/t's from treadwright so my question is would the 30/9.5/15 make that much of a difference in my performance cause when i got my 235's the guy said your speed will be 5 mph difference but at low speeds it's the same but at at freeway speeds it's about 5 but just wondering if the 30's will make a huge difference being not that much bigger

 

http://www.miata.net/garage/tirecalc.html
use this to see how much the sizes differ. take your 195 and the tires you want. it'll show how much of a difference you will be at. if your not sure what your original size was, look in the door jam on the sticker.

if it's off by too much, you might have to either re-gear your axles and or your speedo gear.