Are you hearing 4" and 4 1/2"? If so I think I can explain why. Take a look at this drawing down near the bottom where it shows rear spacing (back spacing) from the mounting pad to the outer edge of the flange. Not everyone agrees this is where you measure from.
Look at this next drawing and notice how they measure the wheel width... It is from bead seat to bead seat (inner edge of flanges), not the outer edge of flanges.
Since the rim width (rim to rim) is less than the actual wheel width where do you really measure back spacing? You start at the mounting pad but then is it 4 1/2" to the rim flange as shown above or is it the 4" from the pad to the same edge they measure the rim width too?
I don't know the answer but maybe this explains the different measurments and maybe someone can clear the air. I've called wheel and tire shops and been told to to lay a straight edge on the bead seat to measure and another shop said to lay the straight edge across rims edge like shown in the 1st picture to measure.
LOL - I've also been told 4.25, having read all the info on rsracing's website it would appear to correct way is to measure from the pad to the inboard flange (and compared to the chart at the bottom it coincides with my rim mfg specs based on the offset calculations). So all that being said are you saying that a FW 15x7 measures 4.5" from mount pad to inboard flange?
OK, now you have given some insight, I ran 15x8's with a 5" bs and they slightly rubbed on the inside at times. The FW (15x7) never rubbed at 4" or 4 1/2" bs and fit inside the fender lip too. Your 15x8 with 3 3/4" bs will put the tire outside the fender lip and require a jacked up stiff rearend or you'll kill your tires on the outer fender lip. Your putting the entire extra inch to the outside with the 15x8 plus the 1/4" - 3/4" difference of the FW.
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