Biggest tires to fit stock 2010 FX2 supercrew with 20" rims
#1
#2
305/55-20 (33x12.50) will fit, but some with aggressive sidewalls may contact the upper ball joint/control arm because of the width and deep backspacing on the OEM wheels. You wouldn't have to worry about this with a slightly narrower 285/55-20 (33x11.50) tire, but it's not a really common tire size.