I'll join the chorus here. When Peter drives Millicent he gets 11 MPG -- going 53-54 mph. I generally get around 10 or 9.5, going 57 or so. When I feel pressed for time and go top speed of 63-65, fuel mileage drops like a stampeding herd of rocks.
Engines generally get better fuel mileage at lower RPMs, when all other factors are the same. This is a simple matter of fewer combustion cycles per mile covered. So this suggests that changing to longer-legged gearing would improve fuel mileage at all speeds. But note that I said "generally", because...
...Engines run most efficiently at a certain RPM, determined by factors from bore/stroke ratio to cam-lobe profiles and timing to turbo-charger specs. So it is possible to get worse MPG at lower RPM. Some truckers learned this when the national double-nickle speed limit was imposed back in the day. The smarter ones corrected this by driving in 12th gear instead of 13th, bringing the engine back into the efficient operating range.
It is also possible that your engine will not have enough power at the lower RPM to keep the bus moving at the intended speed. Then you would be driving back to the shop in 3rd gear to have the old ratio re-installed, since you don't have a 12th gear.
Regardless of the details, more speed will always burn much more fuel, because of the "square of speed" that was mentioned above. Ask any Bonneville racer who makes run after run at, say, 240 in a vain pursuit of a 241 record. That last click might require an extra 50 HP.
And I have personal experience with Millicent: Every September I drive down I-80 from Donner Summit to the Sacramento valley -- a stretch notorious for fiery truck crashes in the old days. There are two or three stretches where it is vital to manage braking-capacity and speed, to avoid overheating the brakes. To maintain a legal 55 I must use the brakes quite a bit, and they overheat. But I can stay off the brakes almost entirely by letting her roll up to 65. At 65 I can practically feel the brick wall of air holding her back.
This matches what I learned from 27 years of trucking cross-country.
But wow, yeah.... (Caution. Trucking anecdote follows.) There were times when I was in a royal hurry to get from California to Colorado for an important delivery appointment, and I would set the truck on 76 MPH pretty much from the moment I entered Nevada, and just steer and grin until I turned off at Laramie, Wy. The computer tracks of those trips were automatically red-flagged in the office, but nobody ever said a word. Sometime fuel is cheap. I doubt that happens much with skoolies, though, or we would be driving Prevosts instead.