can i put a 285/75/16 tire on a rim that already has a 265/75/16 tire on it already
It should fit on the wheel fine, but if your suspension is stock it might rub.
The tire can be mounted to that rim. I would not recommend it. Especially if you are only putting it on one of the wheels. It is two sizes wider. You can have clearance and handling issues. Stick to the tire size that your vehicle was equipped with unless you are using custom rims.
