18x9 front - is 255/35 okay or 245/35 245/40.

 

Okay for what? What are you running on the rear. You want to maintain a factory(ish) relationship to the staggered stance that the car is set up for.

 

Dude what else? to put on a 18x9 wheels. I know I want to maintain the factory spec. That's why I'm asking this question. Mostly use 235 or 245.

 

C2 or a C4? You don't want to burn up the diff. if you're a C4.
255/35/18 is a perfect diameter match for 295/30/18. Both diameter's about 25 inches.
Im running 255/35/18 on the front on a 10 inch wide rim. It will be fine/fit on a 9 inch rim as well. Just make sure your wheel offset is good so that it won't rub on the strut or fender.

 

C2 okay. I don't want stretched tires that's why.

 

The ideal section width for a 9" rim is 255. However, 245/35/18 F + 285/30/18 R would be the best combo with a 10" rim at the rear. 255/35/18 F + 295/30/18 R would be the best combo with an 11" rim at the rear. Also, offset or ET is a factor. ~50ET for the former, and ~45ET for the latter. Good luck.

 

Thanks man. Gives me an idea.