I am the proud new owner of a 2005 suburban. This morning i hit a spare tire someone left on the road. It destroyed one of my rims and that's a problem because the rims were aftermarket from the previous owner, and as far as i can tell discontinued.

I am trying to find new rims but don't know the offset i need, the specs i know are as follows:
5. 25" backspace
9" rim
Lug size M14 x 1.5
Lug pattern PCD 6x139.7
and the brand new tires i had just put on are 275x55r20s

My google-fu is not to great because everything i find is giving me a range. Any help from this fine community i now find myself a part of would be much appreciated