1 - Go to
http://www.1010tires.com/TireSizeCalculator.asp
Find info on the ORIGINAL size wheel & tire that came with your car, in the manual or on the door sticker. If it lists more than one, just select the first one. Enter it in the calculator. Or you can select the make/model on the calculator and let it guess at the stock size.
2 - Take a guess at the proper tire sizes and enter them into the calculator. It will tell you the amount of error on the speedo, and also if you are more than 3% off the original tire diameter. If you are, it will screw up your ABS and tires may rub.
If you are not familiar with tires, guessing will take you forever, so I ran it for you. You're welcome.
My runs using this tool say:
255/30/20 Front, 265/30/20 rear
You have a 75% chance of this all working. With 20" wheels, you run a lot of risks. You really did this backwards. You should verify that a tire/wheel combination will actually work and THEN go buy them. Good Luck, hopefully they fit!!