The 285 will be a bit narrower so not so much stuff will be hitting the body coming off the tires. Still, you will have some tread out past the wheel well opening.
These are 285/758RX18's on an 18X9 KMC XD Addict wheel with a +18 offset so you can see a bit of tire does come past the body line:
A 285 section width tire = approx. 11.2204"
A 305 section width tire = approx. 12.0078"
So the 305 tire is approx. 3/4" (.7874") wider than the 285