I have wheels that are 19x9.5 with a 33 Offset. What's an optimal tire size for this setup? Will I have any rubbing as a result? Lowered on Ark lowering springs.

 

The front will be fine with a 255/30/19, but the rear will rub regardless of the tire size.
If you want to add a bunch of negative Camber to the rear then it can clear, but then you'll be wearing out the inside of the tires quickly.

 

I was told by the previous owner to run either 245/35/19 or 255/35/19. Would you suggest one of those or what you suggested? Also, why would it rub in the rear but not in the front of its a square setup? Would getting rid of my lowering springs solve the rubbing issue?

 

Based on the tire sizes the previous owner recommended I will assume his car was not lowered...or if it was it was very mild, like F Sport springs or RSR Half Downs...but I'd have to guess it was stock height as the tires are much larger in diameter than what the car was designed for.
What size were they running?
Did the previous owner mention what kind of tires he was running also or if he had to make any suspension modifications? i.e. more Camber in the rear.
And yes, the Ark lowering springs is what is contributing to the rubbing along with the offset.
And to why the front would not rub and the rear would...the front fenders and rear quarter panels do not have the same shape and dimensions...additionally the distance between the front wheels (left to right) and the rear wheels (left to right) is not the same.

Without tires, 19x9.5 +33 would be about the max for the front, stock or lowered (depending on how much it's lowered).
Without tires, 19x9.5 +38 would be about the max for the rear, stock or lowered (depending on how much it's lowered).
Also this doesn't factor in additional suspension mods, i.e. Camber arms, or stretched tires...i.e. 255/30/19 on a 19x10

As you can see from above there are several factors that go into proper fitment and making them fit.
If would be nice to know what setup the previous owner of the rims was using on their IS...that will give you an idea of what you can do on your IS.

 

He ran 235/40/19 and thats when he experienced rubbing in the front. It was still lowered using the same ARK springs at the time. Im trying to find out what the optimal tire size would be where I wouldn't have any rubbing and also wouldn't have to go with a stretched setup. Im sorry if Im asking dumb questions, one thing I never understood was the whole wheel and tire setup stuff

 

235/40/19 is a huge tire in terms of diameter.
A stock 3IS tire is 25 inches in diameter...a 235/40/19 is almost 26.5 inches in diameter. No kidding he was rubbing in the front.
He was trying to make the tire as narrow as possible to clear the rear, but didn't account for the fact that the front would hit the fender liner (this is most likely where it was rubbing).
The ideal fitment for a 235/40/19 is a rim that is 8.5 inches wide...he had them on a 9.5 inch wide rim...which technically is still safe by the tire manufacturers specs, but at the limit of fitment.

For a 9.5 inch wide rim ideally you want a 255 or 265...the 265 being the correct fitment width for the rim, but there is no way you're going to fit that on your IS without rubbing in the front and rear.
You could try a 245/35/19, but I think your best bet is a 235/35/19. The tire would have a slight stretch, but still within the manufacturers safety parameters.
It won't offer much protection from potholes or curbs at all, and ride quality would be ok, not great, just ok.
It should clear front and rear with no rubbing...depending on the tire, as not all tires are created equal, if it was going to rub anywhere it would be in the rear, but you should be ok.

I think you probably see why he was selling them now.

 

I think I am going to try 245/35/19 and see how that fits, I just don't want to stretch as I'd rather have normal tires and back to stock suspension as opposed to the other way around. If 245 is still stretched, I might go for 255 and again--just go back to stock suspension. Would spacers solve the rubbing issue at all? Or would my best bet be to go back to stock ride height

 

Spacers would make it worse as it will push the wheels further out and reduce the offset even more.
Your best bet would be to go back to stock height if the new tires rub.

 

Any update on this? Were you able to run the 245/35/19 fine? I'm looking at a wheel with the exact same specs on the rears ... 19x9.5 +33 ...

Thanks!