Below a quarter tank - how bad?

Automotively minded Dopers, how bad is this?

I’ve always heard it’s bad to let a car go below a quarter tank between fill-ups. Something about increasing the chance of the dirt collected in the gas tank getting sucked into the fuel system, where the dirt can cause clogs. Also something about “vapor lock”. I’ve never experienced this myself, nor have I heard this happending to anyone I know. OTOH, it’s not the sort of thing that comes up in conversation.

Lately my life has become even crazier than usual, and I’ve gone longer between fill-ups as a time saver. I’m often running late to an appointment a long drive away, so I only find the time to fill up at the end of the return trip.

I’ve gotten down to an eighth of a tank many times now, and a time or two even down to “Empty”, before I’ve filled up. How bad is this


for my car? The most gas I’ve ever put in the tank has been 10 gallons, and the tank is rated at 11.9 gallons. Did the manufacturer make the “Empty” setting on the fuel gauge equal to 10/12 full to protect myself from my own foolishness?

Are the risks of an empty gas tank worse for older cars?

Thanks for the Straight Dope!

Vapor lock is a thing of the past - it was a problem in older, carbureted engines with low-pressure fuel systems.

Most cars have the fuel pump sucking from the bottom of the tank already, so letting the level get low doesn’t increase the amount of debris you’re taking in (and I doubt there’s much stuff floating around in your tank anyway).

I wouldn’t worry about it. I’m very anal about taking care of my car, and I often wait until the tank is close to empty before refilling.

I run mine down to empty all the time (at least that’s what the gauge says, I still have 1 or 2 gallons left.) I don’t think it’s bad for it either, unless possibly you’re pumping very dirty gas. I change my fuel filter and/or other things when I notice my mileage per tank of gas dropping.

I did have to replace my fuel pump at around 150,000 miles, I think it was probably just at the end of it’s normal service life. But possibly the pump is cooled by the gas and could have burnt out quicker if it didn’t get enough gas sometimes…

I’d say these problems were probably more endemic to certain makes and models and that they’re largely from the past, a non-issue for most of todays builds. Also worth considering is that a gallon of gasoline equals 5.8 to 6.5 lbs., depending I believe on the octane and temperature. So, if you’re always lugging around a quarter tank that’s never anything but excess baggage, say 25 to 50 lbs depending, that’s going to come at a certain cost as well.

It can be a problem in older cars, especially if the tank is not sealed very well. If you live in a colder climate, condensation can occur in the air in your tank - which can lead to water in your fuel. Since water is more dense than gas, it will sink to the bottom and be the first thing to get to you fuel line. Keeping the tank full means less air and less condensation.

That said, I haven’t heard of any problems in cars that were made after the 70’s. If it does happen, there is an additive called HEAT that will take care of water in your gas.

On vehicles with an in-tank main or boost pump (most of them these days) the pump is often designed to be cooled by the fuel. A burned out fuel pump is a not uncommon result of running the tank too low.

If an in-tank boost pump fails, the vehicle may work fine in cool weather, and with a full tank, but indeed CAN vapor lock at a lower fuel level, as the high pressure pump may be located high enough that the suction line cavitates. Although the electric fuel pump is not bolted to the engine block as in the carb. days, there is still quite a bit of electrcially generated heat to cause problems on a hot day.

The rationale I heard for not letting the car get below a quarter tank is that you might have to get gas at a really inconvenient, as opposed to a merely inconvenient, time, or you might even run out of gas on the road.

I’ve always heard that a full tank gets better gas mileage than an emptier tank. I have absolutely no idea to what extent this is true or just bullshit. But I thought I’d throw it out there for others to confirm or dispute.

For this, a bottle of generic gas additive or gas system cleaner will clear things right up.

I use one bottle every month or two. Improves my milage, I think.

The extra weight of the fuel in a full tank requires some extra energy to haul around. Off the top of my head, I can’t think of the factors that might offset this.

Well, you can drive farther with a full tank.

I don’t see how hauling around the extra weight would improve miles per gallon though. Sounds like BS.

So all this time I thought I was saving money by filling up with a half a tank left, I was actually slightly wasting it? I always thought that the extra weight wouldn’t help, but I thought there was some mechanism by which more gas in the tank meant better mileage. I’ll be sure to dispell this myth next time some well-wisher gives me that advice.

So where does this rumor come from? Has anybody else heard this, or is it just my group of friends and family?

Does a low tank contribute to gas line freeze.?

The only way having a full tank on each fill up be more economical the the alternative - i.e. not dragging a load of dead weigh around half the time, is when the reduced economy is offset by reducing the distance needed to travel to a petrol station more frequently. How far you have to travel to obtain fuel being the significant factor.

I invariably run my car to the last dregs (4~5L) before refilling completely as I am far to idle to spend my time visiting petrol stations any more than absolutely necessary.

The petrol station I use is half a mile from my home.

Not a bad idea on older cars- but on late model cars you’re doing it too often.

This was once true, but not now unless you have an old car. Back then, tanks were vented to the air; moist air entered as you used gas, and the moisture condensed on the inside of the tank.

Nowadays, tanks are sealed, and air enters through a complicated vapor recovery system. You may have seen the coffee-can size canister under the hood; that’s a charcoal filter, and there’s yards of tubing going everywhere. I have a foggy idea what it does, and I have no idea how it works.

The bottom line is, water vapor no longer gets into the tank. When you open the cap to add fuel, the fuel pushes out air, and then you seal the cap again.

My mechanic told me last week that the first wave of the sealed-system cars are starting to have problems, and he had to buy a $2500 machine to track down problems. If the first few tests don’t find the glitch, the machine will blow smoke into the spiderweb of tubes, so the mechanic can find the cracks. :rolleyes: Oy.

Here’s an answer: Re: Do cars get better gas mileage on a full tank or a less empty tank

According to this guy here(fourth post) when the fuel level is below 1/4, the fuel sloshes around in the tank and leaves the pump pickup dry. This can cause misfires and drop engine performance and mileage.

Good cite. But I’ll point out that ones personal body weight has a shitload more effect on the gas milage than the tiny amount that could be caused by the weight of the gasoline. Other that older tanks- full or nigh empty, it’s the same milage within any normal significant figure. Certainly 'stop and go" driving will be bad for milage, so stopping every 1/4 tank to re-fill can hurt your milage.

Lots of cars with pure voltage driven analogue gauges aren’t very linear. So, you top off your tank and the needle is above the full mark. You drive 275 miles and get the needle to half. But for the second “half” you only get 150 miles before hitting empty. Well, it’s obvious, then, that you have great fuel efficiency when the tank is full!