285 is the width in mm. What is the next number? That is the number that is a percentage of the width which you'll multiply by two and add the rim height for total tire Outer Diameter tire size....a lot of work I know. I hate metric. Go to www.1010tires.com
for a tire calculator but 33" tires work with stock 20s or 17s with no problem on a 2wd or 4x4 Ram.