1971 demon tire sizes for 15” wheels

Uncle Ben said

Rear 255/70/15

Front 215/70/15

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255/70R15? that's like 29 inch diameter a bit too big. 28 inches is what I would consider max.
215/70R15 in almost 27 inches. You do not want to exceed 26 inches diameter in the front.

So the sizes I've used before on 15 inch rims that worked with good effect are:
255/60R15 in the rear (27 inch diameter)
225/60R15 in the front (25.5 inch diameter)

Also I believe 4.25 inch is the max backspace in the front for 15 in rims.

I'm now using 17 inch rims. 17X8 in the front and 17X9 in the rear.