I am thinking about putting 20s (245/35/20 rubber) on my jag. Please tell me what you think. I need help fast!

The load rating of a set of 245/35-20s (even if it's an extra load tire) would have almost 400 lbs less capacity than the stock tire size. Not something I would want to be following on the highway in mid Summer at 65 MPH. At the very least, those are going to overheat and wear extremely fast if they don't blow out first. I wouldn't go any larger than an 18".