The efficiency used to be around 75 percent, now, with the heavier belt, it's moved up to over 90%.
Up in E. Lansing, where GEBE is, there are airplane engineers, using recumbents with windsheilds, claiming over 300 miles per gallon.
Keep in mind, this is the 25 cc, the 35 cc for heavier folks gets about 180 I hear.
But belt drive, coming off a small gear (about the size of a quarter) above a large snap-on ring, (approx 20 inches) driving on the spokes is scientifically more effective than two semi-similar sized gears on front and back.
I'm liberal arts, but the ratio seems multiplied by 10 at least.
The ONE big warning is, the bike/engine seem to meld physically to a single rider, the belt will easilly last for thousands of miles, until some fat college buddy jumps on and doesn't hear the engine "talking to him", shredding said belt on the first major incline.
I use a cruiser single speeds for my demo rides, don't let anybody on Rocinante.