Thanks for the input FORDEM.
From the 215/70-15's (26.9") to the 225/75-15 (28.3") is 5.2%.
Obviously it goes through the gears pretty easy with the well worn 215/70-15 tires. It also turns 3000 + rpm's at 70mph with these small tires. I would hope to still have 2700 to 3000 rpm's at 70 when finished. I hope 5th will still be useful. Now if I could find some 5.12's...
Also, I am figuring in tire weight, which are just big rubber flywheels. I am looking at a pretty street (all season) tire, which come in at 22-25 lbs. More aggressive tires can weigh 26-32 lbs. It all adds up.
Toyo AT-II, has a pretty light AT tire at 28 lbs. General Grabber AT tire is 30.5 lbs. I am leaning towards the Cooper Trendsetter SE all season at a pretty light 22 lbs. I can have 10 lbs per tire lighter if I shop smart.
As you can tell I sometimes think things over too much.
I don't know if Discount Tire will let me try two different tires for fitment & rpm's...