For short trips, where you may only be on the highway for a short period of time and few miles, it is more efficient to just use the battery (Normal mode.) If you want to maximize your gas fuel economy you need to get the gas engine up to full operating temperature. This can take as long as 20 minutes of driving on gas using Hold mode, you want to get the engine coolant temperature up to approx. 190F. If you truly want to maximize your fuel economy when using gas (or electricity) be sure your tires are properly inflated (many prefer to set the tire pressure to 2-3 lbs over the recommended 36lbs.) Also, keep your speed to 65mph or less. Driving fast takes more energy whether running on gas or electric.