Wheel and Tire Selection
If you are thinking about putting different rim/tire combo on your V6 Mustang, you might want to consider the information presented below.
The '94-00 base Mustang came with 205/65/15 tires on 15"x7" cast alloy rims (about 37 lbs per wheel). Starting in '00 you could also get 225x55/16 tires on 16x7.5" forged alloy rims (about 43.5 lbs per wheel). That combination became standard equipment in '01. Ford apparently decided that the 99+ cars could handle an additional 6.5 lbs of wheel weight without any noticeable change in the car's performance. But there is no question that heavier wheels produce a performance loss. They are harder for the engine to turn, for the suspension to control, and for the brakes to stop. Heavier wheels slow acceleration and produce longer stopping distances.
Tirerack.com measured the performance of a BMW 3-series sedan with its stock 16x7 wheel/tire combo and then with a 17x8 wheel/tire combo that weighed 7 lbs more per wheel. The addition of 7 lbs of wheel weight resulted in a deterioration in the car's handling, a measurable increase in its lap times and a 1.7 mpg drop in its average miles per gallon. Conversely, when they installed lightweight rims that reduced the car's road wheel weight, the car's performance (gas mileage, etc.) improved.
Wheel Width vs Tread Width
In addition to considering wheel weight, you should think about what tire tread width will work best on your new wheels. The fact that a tire will fit on a wheel is no guarantee it will work effectively on that wheel. Radial tires are very sensitive to wheel width. A good rule of thumb is to select a tire with tread width that is within ± 1/2 inch of the the wheel width. If you are serious about corner carving, you should lean toward a tread width that is equal to or a bit less than the wheel width.
My Wheel & Tire Selection
In June 2005 I removed the 15x7" cast alloy machine-finish wheels with 225/60/15 tires that had been on my car for five years. I replaced them with new 16x7.5" forged machine-finish wheels that I picked up cheap on ebay.com, and some Bridgestone Potenza RE750 tires. Not a very exciting upgrade, I'll admit. But the issues are the same regardless of your choice of wheels and tires.