rim backspacing

[Bostonfj60]

So I'm trying to figure out what rim backspacing I need for my 35's. I need to purchase a new rim for my 5th spare, but can't remember what the backspacing is?!?!? I believe the rim is a 15"x 8,

Anyone have any ideas? I know the stock backspacing is 3.5"....

Thanks!

JR

 

[JJS]

I have 12.5" wide tires on steel wheels with 3.5" backspacing. These do clear the rod ends. I'm going to narrower rim and tire next, though. This is also on a 40, but the same should apply to a 60.