I’m not sure what your logic is here. If an FBO and flight school share the same owner, or are even the same business entity, you’d think they would put in the second fuel for their old 172’s if it saved money.
They don’t, so I suggest it’s likely not worth it. I could be wrong, but if you ever do the calculations on fuel cost as a portion of TOTAL operating cost including depreciation, and actual repairs, you’ll see why so many people who didn’t need body on frame trucks kept buying them despite fuel price spikes. You gotta be on the extreme far end of the mileage chart before fuel costs really matter. One reason I’ve been suggesting forever to dump all the stupid mandates and greatly raise fuel taxes so the market will favor efficiency.