What's the difference in rear tire size for the G between 275/30/20 vs. 275/40/20?

Can I use the fatter tires?

 

the only difference is how thick the tire is.. the width between the rim and the tread of the tire. ou can run it just fine.. it just means you have more tire... i think i am running 285/35/19 on the rear.... if you are running 20inch tires i would recommend going with the 275/30/20... but it also depends what you are dropped on....

 

You gotta keep the same diameter on the tire, otherwise your going to throw off your speedometer. So if you went with a 275/40/20 vs the recommended 275/30/20 your going to have more sidewall, yes, but also a bigger diameter tire.

for example,
The recommended tire is 275/30/20. We want to put 275/40/20 for more sidewall.
According to a tire size calculator, the speedometer will read 12.3% too slow, so when it reads 60mph, we are actually going 67.4mph.

Thats not good.
http://www.miata.net/garage/tirecalc.html

 

Good to Know. Does 285/30/20 make a difference in speedometer mikeyboy?

 

^ Used the link and found out it actually better to use 285/30/20 for rear but will this rub on fender or sidewall? What if its dropped on Eibach springs will 285/30/20 be a problem with sidewall and/or fender?