If you go with 18"s, and go with mfr size specs, you'll need to calibrate your speedo/odo. That's assuming you were originally equipped with 255/50r20 , and go to original 245/60r18.
If you go to 255/60r18, the tire dimensions are basically the same, except for the sidewall height. The 18's will have 1" higher sidewall. Width, diameter, circumference/revs per mile are the same.
If you haven't looked it up yet, the wheel specs for the 20" is 20x8.5, 5 on 4.5, 2.5" hub bore, 44mm offset; 18" is 18x8, 5 on 4.5, 2.5 hub bore, 44mm offset.
I'm not sure what effect the 1/2" narrower wheel will have on handling, if any. Someone with more knowledge about that will have to chime in about that.
But you've probably already researched all this yourself. I just had a few minutes to kill at my computer at work...![]()