Well you will get better clearance with the 10 rim with 4.5 backspace then the 9" with 4.5 backspacing because the tire itself will be wider on the smaller wheel. The tire will ride 1/2" further past the wheel on each side on the 1" narrow wheel. But the 1/2" will not matter. The same tire will give the same clearance for a 10" wheel with 5" bs as a 9" wheel with 4.5" bs.
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