stop giving me stupid tips on how to save gas

yes media, i remember two years ago all the helpful tips you gave to me on how i can save gas. i don't blame you for bringing it all up again, because we americans have terribly short memories. honda civics were the best selling vehicle for about two months in the USA before it reverted back to the ford f150 full size pickup truck. but if you are going to give us tips, at least don't give us stupid ones. here's some text i just received in an email

Only buy or fill up your car or truck in the early morning when the ground temperature is still cold. Remember that all service stations have their storage tanks buried below ground. The colder the ground the more dense the gasoline, when it gets warmer gasoline expands, so buying in the afternoon or in the evening....your gallon is not exactly a gallon. In the petroleum business, the specific gravity and the temperature of the gasoline, diesel and jet fuel, ethanol and other petroleum products plays an important role.

first of all, a gallon is ALWAYS a gallon. the amount of mass a gallon of some material contains can change with temperature, but a gallon is still a gallon. density goes up as temperature goes down, and with gasoline this means more energy.

the thermal expansion coefficient of gasoline is .000950 per degree celsius. that means for every one degree celsius the temperate drops, you get .0950% more energy from a gallon of gas. let's assume that daytime temperatures go from 70F to 100F, the difference being roughly ~17C. That means you'd save about 1.6% if you bought your gas at 70F instead of 100F.

so yeah, you might save a few cents. but if you re-read the original text, you will see that it says the gas is stored in large underground tanks. once you get below about 4-6ft, the ground temperature is extremely stable, so none of that math matters anyway because the gas is always the same temperature.

and on top of all that, you've assumed that the pumps are accurately measuring volume in the first place, when in fact they are most likely measuring something else (like flow) and using that to approximate volume.

here's another bit of text from the same email

One of the most important tips is to fill up when your gas tank is HALF FULL. The reason for this is the more gas you have in your tank the less air occupying its empty space. Gasoline evaporates faster than you can imagine.Gasoline storage tanks have an internal floating roof. This roof serves as zero clearance between the gas and the atmosphere, so it minimizes the evaporation. Unlike service stations, here where I work, every truck that we load is temperature compensated so that every gallon is actually the exact amount.

the evaporation claim is debatable. you have a fuel cap that is pressure tested when you get your emissions checked. the reason they are testing it is to make sure you are not leaking evaporated gasoline fumes into the atmosphere. so yes, gasoline does evaporate, but it stuck in your tank. and yes, if your tank is completely full, there is no air for the gas to evaporate into, but as soon as you start driving there is.

and here's another think to think about. if you have a 15 gallon tank and you never get below half full, in the course going through 15 gallons, you are, on average carrying around 11.25 gallons of fuel. gasoline weighs a little over 6lbs per gallon. that's about 70lbs of fuel.

if you let your tank go all the way to empty before you fill up, you're averaging 7.5 gallons, or 46lbs of fuel. that's about a 25lb difference. are you sure that you are losing enough gas through evaporation to make of for 25lbs of weight?

on the other hand, some similar "how to save gas" articles suggest ONLY filling your tank half way, in which case you would average about 3.75 gallons of gas or about 23lbs, a 42lb savings. is it worth it? how can both of these claims can be true? well actually they *could*, but are they? i have no idea, but my guess is probably not.

if you REALLY want to save gas here are a few simple steps that ACTUALLY make a difference and they should all be really obvious to you already.

1. drive less.
2. drive slower. when your speed double, wind resistance quadruples.
3. reduce the weight of your car, preferably by driving a smaller car
4. coast as much as possible (coasting in neutral is illegal in Virginia and possibly other states for some reason, see VA§46.2-811)
5. avoid using your brakes. do this by taking your foot off the gas sooner and coasting to a stop when possible, or not accelerating when you know you're just going to have to slow down again soon. this is really just restating #4.

that's it. that's how to save gas. if you want to be obsessive about it, check out ecomodder.com and people there will be more than happy to explain to you how to take this shit way to far and squeeze 100mpg out of your 3 cylinder geo metro by building a fiberglass boat tail and wheel covers.


note: i wrote that sentence first and found the picture second.