Well, I contacted Rocky-Road.com and am waiting for the guy to call me back. I gave him the measurements that I believe to be right (15x7, 40mm offset, 5.07 backspacing) and he's calling "the guys" to verify the backspacing. He's thinking it should be something more like 3.75 (I'm ballparking this number because I can't remember exactly what he said). While I'm waiting, does anyone know for sure if that number is right? Or know of a web resource I can use to verify it myself? I've screwed up once already getting new tires/wheels (-19 offset! Looks good, but rubs), and I'm hoping not to repeat. Wish me luck... I need it.
EDIT: Found 5.25 as the number for backspacing on XOC. Basically the same, eh?