for good quarter mile times you would want a rather small, wide rim, with a tire that has a large soft sidewall. this optimizes traction. with a 20" wheel, you're going to be looking at minimal traction due to the lack of sidewall.
Weld Draglites, Bogarts etc.. Skinnies on front and an 8-10 inch rim on back depending on what tire your running, go with a Bias-Ply Drag radial or Slick! And YES the best rim sizes are 15-16 you don't wanna go any bigger than that, lots of new edge Cobra Dragstrip guys run 15's even though stock it's 17!
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