Billy,

Correct me if I'm wrong, but I think what you meant was:

The diameter of the tire is more important than rim width.

I use a 285/75x16" for a spare wether I'm running my 33/12.50x20" or my 285/75x16".

They're all just slightly under 33" diameter, no matter what the width.

Granted, I may have to rotate the spare to the front if I get a flat...but a tire of different width & the same diameter will usually get you to the nearest tire place.