Keep Your Tires Inflated And Your Tank Filled. You can lose up to ten percent of your fuel efficiency if you drive with under-inflated tires. Fill up your tank in the morning (colder, denser gasoline) and keep it filled (less air to allow for evaporation.) A website calculates your savings for you here. There are some more great tips on saving gas here. And may the wind be always at your back.  ¶ 3:41 AM