It's 4in. That is the sweet spot. More and you hit the inner fenders, less and you hit the body. If you decide that you want something other than the norm, trekoutfitters will do any backspacing you want for $60 a wheel and 10% off if you mention this site. I am running 3in backspacing now. Would not recommend it to you.
2009 King of the Hammers
BUCK Wild Racing
Co-driver: Pat Quirk
Rover Tracks www.rovertracks.com
DJ Safety www.djsafety.com
West Coast Rovers www.westcoastrovers.com
Reel Driveline www.reeldriveline.com
Made it further than half of the other guys, but the Hammers won.