Any engine in any car made, with very few if any exceptions, will get better mileage at lower RPMs, same speed and conditions. If a hill is too steep then downshift, assuming only top gear for comparison.
Best torque is always too much power to maintain any reasonable speed, unless you're talking 75 plus MPH. A 28 hp early type 1 vw would probably be most efficient floored at 75 MPH (if it would do that on flat ground
).
You should see a decent improvement in MPG with the larger diameter tires, but it may take a few miles to break the new ones in so it might be awash until then.
regards
mech