My gawd you people are making this hard. Way harder than it needs to be (Although that might be the idea
).
Going with your estimate of 631 revs per mile for the 32" tires (which seems low - per their web site, Toyo says their Open Country AT 265/75R16 gets 656/revs/mile)
AND your 315/75R16 Toyo's get 595/rev/mile - per the Toyo site
You can divide the GY revs by the Toyo revs to get your mileage factor ratio. In this case (assuming that the GY calc is correct - doubtful) you get a milage factor of 1:1.060504202.
So for every mile indicated on the odomiter you have actually traveled 1.060504202 miles. This difference in mileage does add up quicker than you think.
For any given tank of gas I get about 280 miles indicated on the odomiter. My BFG's have a mileage factor ratio of 1:1.053244592. In my case this means that in actuality I have traveled 280 miles multiplied by 1.053244592 or 294.91 miles.
A difference of 14.91 miles. While not a ton, it does add up to about 53 miles saved per 1000.
I would be willing to bet that the GY 265/75R16 turns close to 650/rev/mile so your mileage factor ratio sould be closer to 1.09.
Hope this helps...
