18" size is 235/55R18. ~28.17" tall
20" size is 245/45R20 ~28.7" tall

So your extra diameter eats up a little (not all) of the gearing advantage. Extra rotating inertia also chews up some of your advantage, although the bigger tires will help launch harder. Probably the fastest thing would be to order the 20's, then take them off and put some wide 18's on there.