PPGs and the likes (other gears similar in design, i.e Albins, PAR, DS1) are heavier than stock. I catn remember by how much. But the gears are wider and the teeth usually taller, adding more metal, in turn adding more weight. Your MPG depends on alot (we all know that). Gear ratio, HP (which can net better sometimes) and the likes. My Albins gearset is 1-5, stock ratio, Im hoping to keep about the same gas mileage, if not, oh well thats the price you pay for modding your car.
Sorry for getting OT but thought I would add my two cents here.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX