backspacing on a 2001 ford 150

i have a 2001 ford f150 fleetside and I need to know how much backspacing i need to tuck a 20" x 8.5" wheel ASAP. Thanks guys

Well I have a 1999 with 20x8 and 5" backspacing and it's tucks fine up front and there's about half an inch between the wheel and the fender in the rear. So I'd say noting less than 5.5".