I'm looking to get new tires for the IS250 on 18" rims.. Would 225/40/18 work with 265/35/18?

I want the wheel to fill out the wheel space

 

2IS 18" wheels on factory suspension? If so, depending on the tire, 265/35/18 would be fine in the rear. Some tire manufacturer's tires run "fat" though; so a 265 might be bigger than you would want. Example, I have 245/40/18 Michelin Pilot Super Sports on my spare set of factory REAR wheels and they're are wide as a "normal" 255 width tire.

 

I think that the wheels will go and a new suspension, haha. 18 drives it is possible to say goodbye to the spine.

 

With the proper wheel offset you could do 225/45/18 front to fill the gap.

 

As current, the circumference looks kinda small in the wheel area of the car. I want it to fill out that wheel area. How big should I go? Maybe 19' wheels?

 

What size(s) of tires do you have now?

 

225/40/18 in the front
265/35/18 in the rears