Will 20"x10" wheels fit on the rear of a 99 camaro without having to cut fenders or do anything to the car?

 

The wheel requires the correct offset, and the correct size tire on it. Typically, the tires available for 20" wheels will be larger in diameter than the cars stock tires. A 285/25-20 would be the same diameter as a stock Z-rated tire though. As long as you control the outside diameter within reason it should not be a problem. If the car has been lowered, it may become an issue. A 275/30-20 would be about an inch larger than stock.

 

How do these specs look?...

20x10.0 5x120 +45 Offset

275/30/20