So Iv been lying to myself that the slight scraping noise from my rear tires as I go around corners is no big deal. I need to get some new tires and have thought while at it I will look at a solution. So the car is lowered(not 100% by how much) the rims are 20" and the tires are 245/35R20.
I don't know enough about cars but have seen on here some ideas are to go 245/30R20 or 235/40R20, which is the best option? Or is there other ones? The tire scrapes on the outside sidewall. It doesn't leave any noticeable marks on the tire so I don't think it is scraping by much.
Thanks for any advice

I had 245/30R20 on Kings "lows" and they didn't scrape at all, so I suggest that is what you should go for. Also, 245s look better than 235s and will grip a little better.
235/40R20 will be 20 mm more in diameter so 10 mm "taller", and they will still be 4 mm "taller" than what you have on at present, so that wouldn't work.
All figures based on 9" wide rims with +38 offset.
See here to try yourself: http://www.rimsntires.com/specspro.jsp

Get the guards rolled? If you're willing to go to the effort of going two sizes up from the car's standard wheel size you may as well. I have a staggered set up on mine (not intentional, just happened to get a bargain on a staggered set of wheels that are worth 5k brand new [not the ones pictured in current avatar]). Fronts are 20"x9, rears are 20"x10. Yes, 10 wide on the rear. Guards rolled and a tiny bit of the inner wheel arch plastic ground off and it never scrapes unless pulling a full load in the back and going at very high speeds over bumps, and even them it's very minimal, light rubbing.

Yeah 245/35/20 is a reasonably common fitment on 380s but a lot of these owners have had their guards rolled. I'd stick to the 245/35/20 for the extra bit of sidewall it gives compared to 245/30/20. Plenty of tyre choices in that size too.