New wheels and tires
 

I just got some new wheels for my 2014 mustang gt and I was wondering if I will have any clearance issues if I use a 305/35 19 on the rear

How wide are your wheels. Did you get the stock back spacing?

Tire needs to be correctly sized for the wheel ; wheel needs to be correctly sized for the car . . . . so like Glenn said, we need to know more about the wheels you got, specifically width and offset

305/35R19 tire would be looking at something around 19x11 with a 52ET.. And not all 305 tires are created equally when it comes to actual tread width

New wheels and tires
 

I bought them from LMR and they said they are made to fit my 2014 mustang gt but they didn't know about tire size, they are 19x9 front and 19x10 rear and the backspace and offset are not factory

305 is definitely too wide for a 10" wheel

Best way I know to find out what tires fit on what wheels, is to look up the tires on Tire Rack and look at the "specs" tab; check the "recommended rim width" . . . you want your wheels to be at least the minimum width, better yet the "measuring width" up to the maximum width.

The recommendations do vary a bit depending on make / model / aspect ratio; but generally 285 is the widest for a 10" wheel

And assuming the wheels were sized correctly for your car, there will be no clearance issues with 285

You need to consider the wheel's offset when determining tire size too.

A 10"-wide wheel with a 40mm offset will fit flush with the fender lip in the back of a 2005-2014 Mustang, with the rear axle centered (usually done with an aftermarket adjustable panhard bar).

That wheel with a 40mm offset will limit tire width to 285 or a 295 if a Nitto tire is used, since Nitto tends to run narrow.

A 305 on a 10"-wide wheel will "fit" but it will have sidewall bulge which curves up the tread patch, so you don't end up putting down a 305 contact patch anyway.
I've run 305/35/20 tires on 20x10 wheels, and I have many pictures of that fitment if you want to see. Not doing that again, it's a waste of money.

So for your application, I'd recommend a 285/35/19 or 285/40/19 if you aren't lowered much (or at all) since the 40-series tire will be taller.

New wheels and tires
 

https://cimg4.ibsrv.net/gimg/themust...35b73ae63b.jpg
https://cimg5.ibsrv.net/gimg/themust...aec6afd159.jpg
https://cimg6.ibsrv.net/gimg/themust...dfea2453f6.jpg
https://cimg9.ibsrv.net/gimg/themust...7443ade6a3.jpg
My new 19X9 19X10 SVE wheels
I would like to see those photos if you have them and my rears are a 42mm offset and I am not lowered at all

My last 20" wheels, a 20x10 set of Niche Milans with 40mm offset, with rear 305/35/20 Nitto 555R tires and front 275/35/20 Nitto 555 G2 tires:
- I had tire rub in the back on the outside of the tires when they hit the fenders over bumps - the car is lowered about 1.5" all around:

https://farm8.staticflickr.com/7814/...3fc7ca72_o.jpg

https://farm8.staticflickr.com/7823/...9ec162c3_o.jpg


I ended up using adjustable rear spring perches so I lifted the back of the car about 1.25-1.5" and they didn't hit anymore:

https://farm8.staticflickr.com/7846/...317b2c8d_o.jpg


This is a previous set, front 19" and rear 20" AMR wheels. The rears were 20x10 with 48mm offet, same exact rear tires as the set-up above, the rears tucked into the fender perfectly:

https://farm8.staticflickr.com/7836/...4237f3ea_o.jpg


And a set of 305/40/18 rear tires on 18x10 wheels with 42mm offset, also no issues with rubbing:
-the sidewalls bulged a bit since 10" wheel width isn't ideal for a 305 tire, and the bulge can be best seen in the last pic:

https://farm8.staticflickr.com/7815/...d472a0a8_o.jpg

https://farm8.staticflickr.com/7821/...0fa6ce6b_o.jpg


I'm currently running 285/40/18 front tires on 18x10 front wheels, and 305/40/18 rear tires on 18x10 wheels.
I also have another set with the same size tires on 18x9.5 wheels all around :)

I kinda like changing wheels up a bit :)