I heard something form my dad, he said he had read somewhere that If you don't match the profile and the width correctly, then it will make your speedometer inaccurate?

I think he said that you have to keep it even like if you have 195/55 you have to goto 205/50. But if you go to 195/50 it will screw it up?

I have no idea, someone correct me please..

 

Check out this link. It should answer all your questions. http://www.miata.net/garage/tirecalc.html

 

Wow dude thanks alot, that solved all my problems!

 

For easy tire diameter calculations, use this formula. While different brands have slight variances, this will get you close. Section width X aspect ratio (in decimal fraction), X two, divide by 25.4, + wheel diameter. Stock P5/ES example: 195 X .50 X 2 divide by 25.4 + 16 = 23.67". Also when going "plus sizing" on your wheels, a general rule of thumb is to add 10mm to the section width and drop "10" on the aspect ratio (I.E. go from a 50 series to a 40 series) for every inch of increased wheel diameter. Again, this gets you close most of the time, but it's not exact in every case.