You can't calculate it like that though.
MPG is based on the engine's efficiency and MAF reading at said rpm range.
Not how many tire rotations per mile.
You can't even use the same engine rpm for a direct computation because said car is now going faster and incurring more wind drag.
Also what you are trying to calculate is steady state cruising MPG.
What about the rest of the time when you are using more accelerator pedal to speed to car up?
What about stop & go traffic?
What about city driving?
If definitely takes more fuel to accelerate then it does at steady state cruising. Exponentially more actually. The cruising fuel demand between a 300rpm range is negligible to none. I can post MAF numbers to show that if you like. If the MAF is reading the same airflow it is delivering the same fuel trim. Period.
So even factoring out engine efficiency at said rpm and wind resistance your final numbers are still incorrect.
I would guesstimate the real final number would be within a meal at your favorite fast food store apart. That's even if you stay out of the throttle and accelerate slower.
Then factor in the price to change out the gear and you would never recover those costs in a decade. ($300 just for parts)
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The difference in final gearing is roughly 5.1% which is 2941rpm vs 2782rpm at 70mph. That's just a 159rpm swing. You will not see a MPG difference in that small amount of rpm change.
