My friend has a 2007 mustang with some aftermarket rims. They are 20"x9". I know you're asking why should I add 20" to my '94 stang, but they look so beastly. Now I was wondering, will it mess up my breaks if I add them to my mustang?
Yes, you'll def. need spacers. The 05 rims suck way into the wells on our cars. I had a buddy with a set of stock 05 GT wheels on his 95 vert and it looked like ass!
Can't you find a nice 17" or 18" wheel that will look correct on your car?
A 20" wheel looks good on the 05+ Mustang because their wheel wells are just so much larger that they can make the wheel look correctly sizes for the car.
On a 1994, they will look huge and even look like it's lifting the car up some. I think 17-18" is the max for these cars to visually look good.
And in terms of messing up the car. The heavier wheels will make your car slower, take longer to stop and you'll use more fuel.
Plus you'll need spacers of an inch or more which may add a safety issue as well.