Well most vehicles have about a 3% difference between actual speed, and what your speedo says...with the speedo reading slightly higher. In my case, after the 60 series tire, my speedo is dead accurate (GPS tested) upwards of 80mph.
You won't need new wheels and you won't need suspension mods if you want to go to a 60 series/ratio in the 275/60/20.

Here's a thread by Tylus who recently put 275/60/20s on his wife's 2wd EL:
https://www.ford-trucks.com/forums/1...lli-tires.html