measure the circumference and divide by 3.14

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fwiw i think the 9 and 10 inch drums look quite a bit different from the outside. i want to say 10s are smoother and 9s have little fins/ribs..., but i could be wrong on that. might help you during your investigation

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1999 xlt 2.5l 5-speed Supercab 159k
83 diesel 2.2l na
06 F250 6.0l psd
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