I would verify that the 20" tires are truly bigger in diam than the stock 17 or 18" wheeled versions. I know in the past, 235-85, 265-75 -16's, are the same diam as 225-70-19.5, 265-70-17, the 245 and 235 width 17 and 18's ford and dodge were using are also 31.5" in diam, so no tire diam change in gearing per say, just a difference in rim diam and sidewall heights.
The brochure or online specs should state if the 20's are a different height overall than std.......of course, the wording is "SHOULD".........
Marty
92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
2014 Chevy 1500 Dual cab 4x4
92 Red-e-haul 12K equipment trailer