Wow, that's more than 3.25" larger outer diameter than stock wheel/tire combo. Your GP can't be happy with you.Speedo at 60mph has to be off by like 10mph. Trans, brakes/abs, etc. all have to be off and under much added strain. Heavier wheels is one thing, but going that far out of specs isn't really something I'd ever advise. I don't know that they make a tire the correct size to maintain the correct diameter even. It would literally be nothing, but a solid rubber band around the rim. It would have to be something crazy like a 245/20-24 or 245/15-24.
Yes, I know it's doubtful you are going for any kind of performance with a set up like that. The most recommended size tire for a GP on 20s is going to be 245/35-20. As others stated, the 255/35-20s the original post asked about are very close to stock diameter and it would depend on offset, etc. Should be ok in most cases though.