Use this tire calculator. I don't think it's perfect, but I'm sure it's accurate. Plus at least it offers you revs per mile.
http://www.miata.net/garage/tirecalc.html
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Use this tire calculator. I don't think it's perfect, but I'm sure it's accurate. Plus at least it offers you revs per mile.
http://www.miata.net/garage/tirecalc.html
Leave the N/V settings alone. Change the "Use VID" to "Disabled" (always), otherwise whatever axle and tire revs you modify in the tune will never take affect. Once you input the proper axle and tire size, the pcm will pretty much do all the rest of the work, including all the shift points for the autos (you may still need to make some minor adjustments to fine tune in the exact target shift point).