Boy, you read some of these guides and wonder where they get their "facts" from. Myth #1. Fill up on a cool day so you get more gas. Fact: the coefficient of expansion of gasoline is 0.00060 per degree Farenheit. That means what you get is what you pay for. Gas stations have their storage tanks deeply buried underground an average of 15-20 feet. At that depth the temperature is pretty constant. So "fluctuations" in the underground temps are uncommon and therefore the gasoline doesn't become denser when the weather is cool outside. Myth #2. Name brands have better quality gasoline. Fact: Different refineries all put out the same thing, gasoline. Oh yeah, some name brands have detergents and "additives" but if you get just gasoline, you get gasoline, period. That's like buying gold and adding lead to give it color. If you want additives, buy it separately. You'll find these "additives" worthless overall. My friend Sal the chemist who usually swears by name brand gas, found one mom&pop gas store was selling hexane heavily mixed into the gasoline. Hexane is a cheap hydrocarbon that lowers the overall quality of gas, it still burns but the energy output is alot less, kinda like waterdowned drinks. I told my friend Sal what prevents that name brand from buying the same doped gas from the same less reputable distributor? He checked a nearby name brand and guess what, he found the same formulation in the name brand. In order to compete with his neighborhood underpriced mom&pop operation, Namebrand also bought some doped gas to "waterdown" and get a profit. Myth #3 You can buy better gas mileage using additives, magnets, or intake turbines, etc. Fact: No additive or gadget maker has ever been proven to get better gas mileage. No matter how many ten to twenty actors with paid testimonials these infomercials throw at you, you will be convinced that this product will work, regardless of what millions of other consumers have already found out, these gadgets and additives don't work. Myth #4 Filling up the tank wastes gas, because you're using more gas by carrying the extra weight. Fact: A gallon of gas weighs around six pounds. In a 15 gallon tank capacity, a full tank is about 90lbs. and a half tank is 45 lbs. National averaged estimated savings is 1-2% per 100 lbs of weight reduced in a car. Keeping the tank half empty would save you 0.5% or 1 penny per gallon. On the flip side, keeping the tank half empty requires more trips to the gas station. Keeping the tank half empty allows more room for gas vapor loss to escape to the charcoal canister. Keeping the tank half empty allows more water vapor to get into your tank to cause rust, because at night, the gas vapor condenses, drawing humidified outside air into the tank. Keeping the tank half empty makes you more prone to the fluctuations in gas prices, because you're constantly buying gas.