the best wheel combo for an s-10 is a 20x10 rear and a 18x8 front

. the problem you'll have with 20's on the front is not the fender wells it is the frame

. when you turn the truck to full left or right the tires hit the frame in the rear (it will actually stop the truck) I have 2001 extreme that is bagged and body dropped with 20's rear and 18's front

and we had to do some serious mods to the rear wells and remove the front. As for a different suspension due to a different wheel you need nothing; you are not actually changing anything on the truck and I believe a 245/30r20 is the exact circumference as the stock tires so no speedo adjustment. I have 275/45/20 on the rear of mine and it's pretty close. so good luck with the new wheels hope this tells you something