Certainly doable. Your speedometer will be off, unless it's corrected. Most of the major tire stores can do that for you though.

 

You won't notice a difference. The 285s are 33.5 inches tall and the 275s are 33 inches. That's a 1.5% difference, so at 100 MPH the speedometer will be off by 1.5 MPH.

 

There are many tire size calculators available online to assist you in your search for information.......

 

here is how to figure the size.

285 60R20
285 mm is the width
60 is the % ratio of the side wall
20 is the rim size in inches
there are 25.4 mm per inch

so take 285 and divide it by 25.4 => 11.22 inches wide tire
the side wall then is 11.22 x .60 => 6.73 inches tall
you have 2 side walls, the top and the bottom, 6.73 x 2 => 13.46 inches of sidewall plus the 20 for the rim gives you the 33.46 inch tire