Like Wayne said, E10 does NOT drop your fuel economy 10%, or anything close. Although it only has about 96.5% of the BTU count of straight gasoline, BTU count isn't everything. Ethanol is more thermally efficient than gasoline and will still yeild about 97-98% of the fuel economy depending on the car's ECU tuning. If you saw a 10% drop, it's because the driving conditions or driving style was not the same.