15X8 w 4.75 backspacing

68 Coronet RT said

I believe the 255/70/15 or the 255/60/15's will fit. I found a conversion chart and punched in my current rim 15X7 with 1/4 inch back spacing with the 235/70/15 and the 15X8 with 4 3/4 back spacing with the 255/70/15's and I believe I'm going to be ok

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I think you're OK with 255's. The one thing to watch for on the SS springs is the spring clamp locations, I remember they were close to the tire sidewall on the front spring segment. Anyway, the 8 inch rim reduces the amount of sidewall as compared to the 7 inch.