I have a 2014 IS350 Fsport AWD

I need help on which tire size to buy. I was told to go 245/35/20 in the front and 275/40/20 in the rear. Is this a good size. I plan on getting Vossen VFS2 20"

But I've seen people go 30 or 35 on the rear

 

I had 245/35/19 (I now have 235/35) on the front and rubbed bad enough to wear a hole in the fender liner. I'm lowered on RSR half downs. I would imagine that this size tire on 20's would be trouble.
I have 275/30/19 in the back and haven't noticed any problems (can't speak to 20's). I'm pretty sure that 275/40 would be trouble.

I rarely have more than one passenger in the car.

 

Any issues right now with 235/35 in the front w/ half downs?

 

Any issues with transmission? The vendor told me that if i go 30 in the rear, he said it could effect my transmission.

 

There has only been one instance where I heard it rub - a huge dip at the seam of an interstate on ramp.
I haven't noticed any difference in the transmission.

 

Oh ok thanks. I Don't plan on lowering it that much, would 35 both front and rear be good?

 

I have no idea about safe sizes for 20's. You should do a search and see what others with 20's have done.

 

Yeah mostly everyone i've seen was, 30 in the rear

 

20 is way too big. Stick with 18. 19 if you must.

 

Oh ok, so probably the best route. is 235/35/20 F, 275/30/20 R

26.476 Front
26.578 Rear

 

Just installed my rear wheels. Tire size 275/30/20 and it rubs. Anyone have this issue?

 

I have Rays TE37 Ultra Bronze

Front 19" 8.5 +35 ( 235/35/19 )
Rear 19" 9.5 +35 ( 255/35/19 )

Loooove the fitment man. I suggest you to go with 19"

 

Unfortunately I only saw this thread now, but I would have told you the 275/30/20 is way too big for the rear.
I'm surprised you were even able to shoehorn it onto the vehicle.

I don't think you mentioned what the width of the 20's are, so that will determine tire size also.
As you can see from my signature, I have square 20's with 235/30/20 all around. No rubbing other than full lock on the front will rub on the vent in the wheel well.
It's only slight, and how often is anyone at full lock? Hardly ever, and likely you are going 1kmph when you are.

I'm not sure if your rims are staggered or square, or why you wanted the 275 on the rear.
Maybe you can provide the rim specs, and that may lead to a solution of best tire size.

 

It's staggered
fronts are 245/35/20, 20x9 + 18
rears are 275/30/20, 20x10.5 + 45

I don't know anything about tire or wheel size and that's what people recommended me to get. I went to discount tires and that's what they recommended me. And I just found they ordered me 20x10.5, instead of 20x10 that I asked. They told me if do 265, I should be good.

 

Wow...and wow!! If there was a list of biggest wheels on an IS, you'd be at the top.
I'm surprised the front isn't rubbing. I think it's safe to say lowering the vehicle is out of the question.

If you intend on keeping the 20's, and the front isn't rubbing, then just leave it as is.
For the rear, because the rim is 20X10.5, you're very limited on what you can do.
Going to a 265/35/20 is your only option, and hope it doesn't rub.
And you might be thinking why not a 265/30/20...simply because the tire is too small for a 10.5 inch rim...it has to do with the sidewall height.
The limit for a 265/35/20 is a 10.5 inch rim...265/30/20, the limit is a 10 inch rim.
The bad news is going from a 275/30 to a 265/35 will only reduce the overall width of the tire by half an inch. If you need more than a quarter inch of clearance where it's rubbing, then the 265/35 won't resolve your issue.

If there is an possibility of swapping out the rear rims for a 20X9.5 or even a 20X10, I'd do that first and then get proper tires that will fit.

Does the front rub?
Is the rear rubbing on the fender?