not all wheel manufacturers measure backspace the same way. To make sure, contact the manufacturer and ask how they measure before you order.
Yeah, I may have found this out the hard way...
I ordered my wheels for my '63 wagon in size 17x7 with 4.5" backspacing. I assumed that, since the wheel width measurement is from bead to bead, that the backspace measurement was from mounting surface to bead. At least that's the way I would do it.
Well, I got my wheels and they are measured to the outer lip, not the bead
. Now, after measuring the wheels outside-to-outside mounted on the rear axle and comparing to the minimum measurement on the inside of the wheel houses, it looks REEEEAAAAL CLOSE. I'm looking at 1/2" clearance min on each side, assuming the axle is perfectly centered (which is a leap of faith in itself).
The measurement from bead to rim lip is 1/2". The extra 1/2" per side would have given me plenty of clearance. Now, I'm looking at the probability of narrowing the axle or heating/pounding the outer wheelhouses.