Is It Better To Not Fill Up?

December 14, 2013 8:19 PM

My car weighs 4013lbs (with me in it) and averages 17mpg on my 21.7m (roundtrip) commute. My gas tank holds 20 gallons, but I only ever fill it up half-full. The reason I do this, is because my brain knows how heavy 4 x 2.5 gallon containers of water is, and thinks that not carrying this additional approx 60lbs of weight (1.5% of the 4013lbs) will be better for both mpg and performance (i.e. less liquid sloshing around the tank). Is my brain crazy?
posted by forallmankind to Travel & Transportation (19 answers total)

I don't have the figures, but I think that the savings you get from carrying less weight will probably be more than canceled out by the extra trips to the gas station. Either way though I expect it would be negligible and not worth the loss of convenience. That's just my instinct though, and I welcome anyone who has hard data on the subject.
posted by Scientist at 8:28 PM on December 14, 2013 [4 favorites]

I think gasoline might be ever so slightly lighter than water, but that's kind of beside the point. Basically, you're filling up twice as often as you would have to at a minimum to knock 2% off your car's maximum travelling weight. If the time to stop and fill up your tank doesn't matter at all, then there's no problem with what you're doing, but I would think it would be fairly silly if you were doing a long trip.
posted by LionIndex at 8:28 PM on December 14, 2013 [2 favorites]

If the light down the street catches you and you have to sit at it, you've burned more gas than if you'd made the light. That gas is more than you save doing this half-full thing. It's minuscule. You're making extra stops, wasting a lot of time, for not a lot of benefit.

Here's something else to think about: if you fill up every time as needed, and track it (it's difficult to accurately track unless you're using the fill-up point), you're more likely to notice a sudden drop in mileage and get it fixed in a timely manner. Saving a lot of gas!
posted by notsnot at 8:33 PM on December 14, 2013 [2 favorites]

If this was a significant factor, cars would not have gas tanks as large as yours.

Also, you mention that we're talking about an extra 60 lbs. That's half the weight of your average adult passenger. So... do you also refuse rides to other people, for the sake of your gas mileage?
posted by Sara C. at 8:48 PM on December 14, 2013 [7 favorites]

After taking a disaster preparedness class I've tried to keep my gas tank at least 1/2 full because if there's some sort of natural disaster, it might be a) hard to get gas and b) prudent to leave the area to someplace less touched by the disaster, and I don't want to have 1/4 tank in that situation.
posted by needs more cowbell at 8:52 PM on December 14, 2013 [11 favorites]

You'll save about a nickel a gallon doing this. Whether that's worth stopping at the gas station twice as often is up to you.
posted by payoto at 8:52 PM on December 14, 2013 [5 favorites]

According to Randall Munroe, removing 50 pounds of cargo results in a 0.5% increase in fuel economy.

So, you can buy 20 gallons and drive about 340 miles, or you can buy ten gallons twice and drive about 340 * 1.005 = 341.7 miles. That's the equivalent of getting 0.1 gallons of free gasoline, about 32 cents worth in my town right now.

However, you need to buy gas more often. If your gas station is on a road you'd be driving on anyway, then it'll only add maybe .1 miles and a few minutes to your driving to hit it twice as often. But if you drive more than half a mile out of your way to get gas, you've negated your savings. And either way, if you spend three extra minutes per fillup to earn that 32 cents, you're working for less than minimum wage.
posted by Hatashran at 8:59 PM on December 14, 2013 [16 favorites]

A tank half full of fuel is also half full with air.

E10 gasoline (gas with ethanol added) is hygroscopic - meaning that the ethanol will actually pull moisture out of the air. Octane drops as water is added to the mixture, reducing performance. Additionally, if enough water vapor makes it into the fuel, it can phase change and become liquid at the bottom of the tank.

It's best to keep the tank full - this reduces the amount of water vapor available to the ethanol.
posted by Pogo_Fuzzybutt at 9:04 PM on December 14, 2013 [7 favorites]

The benefit is significantly less on the freeway, where most of the energy is going to air resistance, not weight. For instance, the compact Prius C gets the same gas mileage as the full-size Prius on the highway, despite being 500 pounds lighter. The weight of the gas is a rounding error for freeway driving.
posted by wnissen at 9:11 PM on December 14, 2013 [1 favorite]

The EPA states that 100 pounds removed from your car improves fuel economy by 1-2%.

In the case where you fill to 20 gallons, and refill at 3 gallons, you average 12.3 gallons of fuel in your tank. In the case where you fill to 12 gallons and refill at 3 gallons, you average 7.5 gallons of fuel in your tank, so we're talking about 4.8 gallons of fuel. With gasoline's density (726g/L), that puts it at an average weight savings of 29 pounds.

If we work on the assumption that 100 pounds improves economy by 1-2%, 29 pounds improves your fuel economy by 0.3-0.6%.

Assuming 17mph, 10,000 miles/year, AND assuming that there is no extra fuel used from the gas stops, AND assuming it does not shorten the life of your fuel pump, you're looking at:

Benefit: savings of 1.7-3.4 gallons of gasoline per year.

Cost: 31 extra fill-ups per year.
posted by grudgebgon at 10:01 PM on December 14, 2013 [1 favorite]

The main problem here is inevitable damage to the fuel pump. In most modern cars, the fuel pump is placed at the bottom of the tank so that the fuel cools the pump, and if you run your tank close to empty a lot, the pump will overheat and fail prematurely. The rule of thumb is that you should always fill up when you're down to 1/4 or 1/8 full.
posted by wutangclan at 10:50 PM on December 14, 2013 [3 favorites]

It's best to keep the tank full - this reduces the amount of water vapor available to the ethanol.

Modern cars have closed vapor systems for pollution control. That means that outside air does not freely move between the inside of the tank and the outside. For every gallon of gas you burn precisely one gallon of outside air enters the tank. It does not matter if the tank it half full or near full, the same amount of outside air enters the tank as fuel is burned.
posted by JackFlash at 11:20 PM on December 14, 2013 [2 favorites]

Let me put it to you this way: if you're driving a two ton vehicle* which only gets 17.5 mpg, going way out of your way to try to squeeze out an extra few cents'-worth of mileage is the least of your problems.

I think you'll find that the weight of your car takes up far less of your mileage burden than you probably think. Air resistance is a much, much higher factor, and that won't be affected by the weight of the vehicle at all. Further, the weight differential will only matter if you're accelerating. Once you're at speed, adding weight adds momentum, basically zeroing-out the mileage effect of any additional mass. So it's not simply a matter of figuring out the percentage of weight you're saving and calling that an efficiency gain. It's going to be some fraction of that. As we're only talking about 1.5% of the total, weight, even half of that would be only 0.75%. That's a small enough number that it'd be almost impossible to separate from statistical noise if you were actually trying to prove it existed.

Fill up your tank the whole way. You'll use far more gas going to the station twice as often.
posted by valkyryn at 2:02 AM on December 15, 2013 [1 favorite]

Gotta agree with both pogofuzzybutt and wutangclan, and say that filling the tank is better.

For what it's with, some of the most knowledgeable people on my usual car forum say not to let your tank go BELOW half full, both for the fuel pump and for the sensor that tells you how much gas you still have.
posted by easily confused at 2:04 AM on December 15, 2013

Why this one item? It's just one factor in dozens.

More relevant would be tire pressure and alignment, tire selection, driving speed, moderated acceleration. You are approaching something with the same effect as keeping the dirt off the paint jobs to lower air friction with this load avoidance strategy. Intellectually, it's appealing. Practically, it's irrelevant unless you are in a milage competition or something.

The single best thing you can do to accomplish what you want (more sanity in transportation) is to reduce miles driven. MPG is meaningless compared to passenger miles per gallon and to total miles driven. Those things drastically increase transportation effectiveness, compared to the minuscule improvements of negligible load decreases.
posted by FauxScot at 3:31 AM on December 15, 2013 [1 favorite]

I only put 8 gal in my tank once a week. If I drive consciously, I've found that this gets me through the week. I've also found that if I put more than 8 gallons, I drive less consciously. In effect, I get better mpg if I only put 8 gallons in my tank than if I filled it up. I suppose the psychology is obvious?; and people would tell me to be more mindful etc if I fill up and it wouldn't be an issue. (Like, put in 16 gallons and try make it 2 weeks man!)

I think the fact is that 60 lbs of weight savings isn't really going to help you much. But psychologically, perhaps knowing you have x amount of gallons to get through a week might affect your driving habits enough to increase your mpg significantly.
posted by herox at 4:40 AM on December 15, 2013

How much extra driving do you do to make those extra trips to the gas station?
posted by J. Wilson at 7:34 AM on December 15, 2013 [1 favorite]

Best answer: Is my brain crazy?

Not so much crazy as only seeing part of the problem. Fuel is just under 3/4 the weight of water by volume so its not as much as you think for starters. Also, you are losing economy as follows:

1: Constant speed is the best economy state, accelerating from rest is the worst economy state. The fact that you have to drive into a fuel station (even assuming you get straight to a pump) and then getting back up to speed on the road means you used more fuel than you saved by carrying a tiny fraction less weight.

2: Every single foot you drive for (every other, you know the even numbered) fuel stop is wasted distance that could have been avoided by filling up completely at the odd numbered stops. Every second your engine is idling while you pull in, and waiting to pull out of that fuel stop has lost you more than the 'advantage' you gained by stopping.

3: It's about 1% of the weight. It's negligible. Your truck already does 17mpg, so if you want to get anything approaching economy you need to be more drastic. If you want to care about economy it will be a tiny, tiny difference unless you buy a car or truck that does decent fuel economy.

So you're focussing on the wrong problem. You're convincing yourself you're doing something to help when you really aren't. So clearly you're worried about economy, but maybe focus on actually doing something for economy rather than this placebo of not carrying around a fraction of extra weight.

After all, you could get silly on this weight saving thing and it becomes obvious that it is daft after a while: Take out your spare tyre, remove all equipment from the car you don't use, take out some seats, wear shorts and sandals and only a t-shirt rather than full clothing, take your watch off and leave it at home, make sure you take a dump before you get in your car, make sure you keep your hair short.....!

At some point you need to focus on the big differences - mostly this is driving style or just change the vehicle.

Incidentally, I refuse to believe that running the tank below 1/8 does anything at all to the fuel pump like some people are saying. The Fuel pump runs in a small depression in the fuel cell and unless it is sucking air it has enough volume of fuel with 10% in the tank to prevent it overheating. Pumps don't produce that much heat to damage themselves unless they actually run dry, and do so repeatedly. The applications I use rely on the cars having almost no fuel in every time they go out (race car on track, using automotive sourced fuel pumps) and we also often pump the car completely dry with that fuel pump after every session. So a fuel pump will run dry about 100-150 times through a season and they often last two years before they get lifed out. Usually it is vibration that kills them anyway.

Also, in other applications, I have a small box with two automotive pumps in it that pump in and out out of every race car rather than use the internal pumps - so these two pumps don't even have the volume of fuel sitting around them to keep them cool and they have pumped race cars dry probably 5-600 times (after having been in the race car for a year first). They still work fine.

So if your car's pump is still submerged, then there is enough volume of fuel to keep it cool enough to prevent damage. It's an urban myth.
posted by Brockles at 7:38 AM on December 15, 2013 [3 favorites]

I tend to agree that what you're doing is a placebo, for the most part.

That said, two things. One is that even with a two-ton truck (why on earth are you driving that much metal all over town every day?), you can find ways to save mileage, and the place to do that is a hypermiler forum. Do that keyword with your model of truck and you may or may not find something useful, but keep in mind that these are obsessed "car guys" who love to tinker, so if getting under the hood is of no interest to you, well....

Two. The second thing. Another vehicle. The hybrids and plug-ins tend to have these nifty electronic displays that tell you everything about your vehicle's performance -- because in those vehicles, it matters (as in stranded on the side of the road matters). I know a guy who owns a Prius, a Volt, and a Leaf, and he has something like eight years of paper notebooks on the mileage/performance he gets out of each of them. He does things like not running the wipers during a rainstorm, except when he wants to see, because it eats a little bit of power. This process is called "gamification" and makes driving one of these vehicles -- if you want it -- an ongoing intellectual challenge. That trip to the grocery store -- did you beat last week's? That hill by your work -- maybe if you go the long way around you actually save? And so forth.

If that's your approach, you may be a candidate for one of those vehicles.
posted by dhartung at 9:04 PM on December 15, 2013