I was told they were 3.5", but as I measure it, (from the outside lip like you would normally measure rims), theres 1 3/4" backspace, and on the front its 3 1/4", plus maybe 3/8 material in the middle... (totals 5 3/8"), I don't see anything close to this listed when I search at summit (and only the 4" rims have 1 3/4" BS - the 6" rims have 3 3/8 BS)... so... umm... (maybe these are measured a different way??)
if it helps, the tire itself (outside to outside) is 6 1/4" wide (the outer lip of the rim is 5/8" in from the outside of the tire on the front side & 3/4" in on the back side - which then supposedly makes the rim 4 7/8" -- ?? ).
and in case anyone wants to know, the overall height is 25 1/2".
hope that helps. when you're dealing with skinnies, does it make that much a difference?