I have a spreadsheet that could be easily modified for this work. It already does speed in gears based on tire sizes, and has a calculation for wheel rpm's per mile.
One other factor to consider is the actual rolling radius of the tire, not just the stated radius (actually diameter, i.e., 31-10.50-15), as the actual may be quite different than the stated. You will also get differences from expansion of the effective rolling radius with speed from centrifugal effects on the tire tread, but they shouldn't exceed the unladen (tire inflated and suspended out of contact with terra firma) radius of the tire, unless you have a serious tire belt problem - an incipient blowout from carcass failure.
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