[aerohead]

The loose rule for BSFC is that it will top out at the rpm where maximum torgue occurs for the engine.By streamlining, and cutting drag,the engine has more mechanical advantage between the flywheel and drive wheels and "see's" less load as it's "easier".------------

By putting "taller" gears between the flywheel and drivewheel(s),the engine has to strain as much as it used to,before you did the drag reduction.This moves the engine back into it's "sweet-spot" on the BSFC map--------------------------

For the gearing,you would calculate the new theoretical top speed for the vehicle,and then choose a tranny,or differential,or combination of the two,which would give the same engine rpm the vehicle had at it's original top speed,at the new top speed.-------------------------

If your engine has a very flat torque-curve,all this may be unnecessary,as the island of top BSFC may occur over a very broad rpm range.---------------------

The hitch to just changing the ratio,is that you can kill your acceleration and hill-climbing ability.Adding an "extra" gear may be necessary to maintain all the other performance abilities.--------------------

My T-100 is geared for 156-mph @ redline.The horsepower peaks @ 138-mph,which is where my road load and horsepower peaks(minus drivetrain losses),so 138-mph is probably a reasonable top speed for the truck.-----------

With no gear-matching,I've been able to get a 37% increase in mpg.