If you were to reduce the road load it would allow the engine to over-speed,and also move it's BSFC off it's original island of efficiency.
If,say you did some LRR tires and streamlining,if you didn't match the gearing to the new load,you could lose up to 40% of the mpg benefit by operating the engine in a less efficient domain.
The new gearing would slow the engine down to it's rated power rpm,while raising the terminal velocity to a drag-limited top speed.
Here are some graphics
