Wheels - 17" vs 18"

It doesn't matter what size rims you get because whoever you get the tires from will sell you the correct size for the rims you have so as not to change the outside diameter of the tire. Lowering a car has nothing to do with what size rims you can put on the car.

You're only losing 1/2 an inch on the rim width, but the tire you get can compensate for that by being just as thick as the 7.5" wide rimmed tires you already have.

My only concern would be that you are losing two inches of rubber contacting the road if you don't go with wider tires for the 7" wide rims, which would decrease your traction.

And with throwing on a wider tire to make up for the thinner rim, you may have issues with the side walls of the tires flexing a bit, thereby also reducing your traction and possibly the tire life expectancy.

yes u can put 18's on your stang w/o lowering it, as in yes its possible, but really it will increase the 4x4 monstertruckin look if you dont lower it. id look into lowering it anyways but if your goin w/ 17's it wont be too noticeable of a height difference

The size of the rim does not affect the gap between the top of the tires and the bottom of the wheel well. The car will also sit at the exact same spot off the ground whether you go with 14" or 20" rims.

When you buy tires, if you get the correct size for you vehicle/rims, the side walls of the tires will be lower profile, fat and meaty, or somewhere in between. If you go out and pick your own tire, and ignore the recommended sized tires for your vehicle/rim, then you affect the gap within the wheel wells, how high your car sits, and you can also throw off your speedo and odometer.

Yes, the stock Mustang already has a higher than "normal" gap in the wheel wells, so a lot of Stangers here opt for lowering.

The selection between 16" - 18" rims all depends on how you like the look of the rims on your car. If you don't plan on doing anything performance-wise to your Stang, then you might want to not go any higher than 17" because of the amount of rotational mass present when trying to turn your tires. Meaning, it takes more HP to get those big metal rims moving than if you had a smaller rim that is lighter.

It's all just a matter of preference.