i have '08 G37s and i just ordered Rohanna wheels Rf2. 19x8.5 front and 19x9.5 back. what would be some good tire size on them?

 

What offsets did you order, and did you confirm caliper clearance for your bigger sport brakes?

 

 

Rear wheel edge is going to be about an inch further out in back, so I would consider a 25mm spacer in front to get both wheels similar at the fender edges.
Once you do that I would run a 245/40 front 275/35 rear.

 

What if I run the 225/40 in the front?

 

OE 19 uses 225. That said, 245 fits fine and reduces understeer when driving enthusiastically. Or are you just going for stance?

 

 

225/45/19 should be your OEM, not 225/40, might check what tires you currently have up there.

Also you'll have a different mount going w/ a 225/275 combo, the 225s are a very slight stretch and the 275s are about standard mount, whereas a 245/275 on the 8.5/9.5 stagger would be a both the same sidewall mount. Just a little food for thought

You could run a 225/45 & 255/40 for a more similar mounted setup too!