This is something that's been subconsciously on my mind for a while now and figured bringing it to minds of others to discuss it could potentially leave me with a more informed decision when at the gas pump. Sometimes I fill my gas tank up completely, while other times i'll fill up the gas tank only half way. Other times, rarely, i'll only add a gallon or two at the pump. Sometimes the reason is money. If I find a gas station where the price is super low and my tank needs filling up, I'll add the whole 14 gallons to my Altima, which for most of 2010 gets filled up completely with about $40. Other times when the gas prices aren't that appealing, but need gas and perhaps don't want to make another gas trip visit i'll fill her up half way, so about 6-7 gallons for about $20. <This option seems to reoccur the most, except in the summer months when I usually fill the tank up completely due to driving a bit to the beach. & then there are the rarer times when I need gas but am either dead tired, or dead tired to get home so i'll stop by a gas station and simply add a gallon or two in a hurry. This situation also may occur when there's only one gas station in the area with higher prices. - The reason I felt like starting this thread was simply cause I remember hearing that filling up the gas tank completely is most beneficial for the car, and also in terms of getting the most mileage. If I filled up my gas tank completely on a Monday, it would most likely last me for a week and a half...for $40. While this week I added $20 on monday, needed gas earlier tonight (friday) and added $10. $30 total this week, and then some. Of course, all this is the intended wanted outcome of spending the least amount for gas. $30x4= $120 on gas a month... Iono, just figured i'm not the only one that has a personal mantra when it comes to filling up their car.