I was looking into wheels for my rx-8, and given the neutral handling of the car, I believe 9 inch width front and back would have the best outcome. Putting larger width in the back, and smaller in the front, would just promote understeer. I think that in the case of the rx-8 if larger width back and smaller front were better, mazda would have done that from factory, such as some other cars like the 350z, or the s2000. I'm sure that on other cars it may be necessary, given that they may not have the same neutral balance as the 8. Please give me your opinions on this issue

 

You are right on - the RX-8 handling was tuned for same size wheels and tires front and rear, and going with a staggered setup will only introduce significant understeer. Anyone who goes staggered is sacrificing performance for looks.

If you're really looking to improve handling, you should consider going to 17" wheels! Reducing unsprung weight is a key component of improved handling, and for equivalent wheels (like SSR Comp, for example), the 17x8s are over 1 lb lighter than 18x8, and the 17" tires are lighter than their 18" versions as well.

Regards,
Gordon