There will be
NO MPG changes with gearing. What determines MPG is total
aero-drag as determined by cA*cD. This drag does
not change with gearing nor
power-required to overcome that aero-drag.
For example, about 7bhp is required to push Ninja 250R through air at 100kph. With taller gearing, guess how much power is required for same speed? EXACTLY THE SAME!!! However, since HP = (TQ*RPM)/5252, instead of 8000rpms @ 60mph, using 7000rpms @60mph will require
more torque. That means with taller gearing, you'll need
larger throttle openings to generate same amount of HP and speed. No reduction in fuel consumed there.
What
will improve MPG is reducing total aero-drag. I've found installing taller windscreen (Double Bubble or Corsa), and tucking in as low as possible behind it improves mileage from 60 to 70 MPG. Also drafting big trucks as much as possible on carriageway really helps (Mythbusters did experiment confirming this).
