Re: 20 inch wheels
It depends on the width of the tires in question, depending on backspacing of course, and how much travel you have available. If you can manage, put one of the front tires on a hill until the rear axle is at, or in the air. Then the truck will be on a good angle for you to measure accurately how much extra travel over the 35's you have. Also keep in mind that running close to 40" tires will put added strain on the transmission and drive train components. Upgrading to at least a 4.56 axle ratio would be advised, if you're not there already. Check out Ebay for some lifted F series trucks. there are lots!
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!