Is it true, or just an old wives’ tale that if you let your gas tank run low you’ll loose gas through vaporization?
My mechanic told me to never let the tank get below a quarter but said it was because the last qurter contains all the crap that is in the fuel. He says sticking to this improves engine performance.
That’d make sense, if the fuel wasn’t drawn out of the bottom of the tank, therefore taking all the crap with it at all times.
Perhaps on old cars, but modern cas tank designs are essentially airtight and won’t cause all of the gasoline vapor on top to simply evaporate away. So, essentially, it is an old wives tale. I’m sure that you will see similar gains in fuel effeciency from having less weight in your car offsetting any difference.
That hisss my fuel tanks when I open it after a good chunk of gas has been burned would seem to say the tank has a pretty good seal. Plus I think they have to be air tight because of envioromentle regulations. So I don’t think your car getting the vapors would be too much of a problem. However the nasties in that last part of the fuel should be avioded.
Just to be a retentive jerk I should point out that with less gas your car is (slightly) lighter and therefore requires (slightly) less energy to move. So you might gat an unnoticable gain from running on 1/4 tank all the time.
What you could do to get that unnoticable gain is get a bunch of gas cans and fill them with gas and keep them in the trunk and rig a system of hoses from the gas to keep the tank at a constant 1/4;)
On one hand, you’re lugging around another 50 lbs of gas, and sloshing gas tends to drain energy more than a lumpen mass.
On the other hand, a mostly empty tank is more prone to condensation (in the short term) and airborne contaminants (after chronic usage) - both of which aren’t good for engine efficiency. Fuel filters can only do so much.
The fuel vapors in modern tanks are recycled through the gas station’s vapor recovery system. You don’t get a credit for this, but it does mean that your tank doesn’t vent to the air, except in emergencies, which limits the vapors produced.
as a rough estimate 4oz of gasoline produces 5-6 gallons of fumes, but a gallong is 128 oz. Therefore you are probably losing, at most a few miles per tank to vapors, if you drive around at 1/3 tank vs 2/3 tank, and you may recover much of that from reduced weight.
Why do you say this after GorillaMan’s post?
I’ve heard this said so many times from presumably reputable sources that I am beginning to wonder where the truth is. Personally, I’m with GorillaMan.
Depends on how big your fuel tank is, and how big a vehicle you are driving. You might get some noticeable gains if you drive a Geo Metro, but the saved weight would be negligible on, say, a Kenworth truck. I know that I wouldn’t want to try and drive a semi on 1/4 tank all the time. Also, wouldn’t any fuel savings be negated by you having to go drive and get gas all the time?
Less gas = less weight and resultant (microscopic) gas mileage improvement.
Less gas = more air volume and more moisture leading to increased potential for moisture condensation (water in gas) and corrosion of internal surfaces.
My recommendation - keep the tank close to full, at least in the winter, if you live in cold climates. (Advice from someone who has suffered the dreaded gas line freeze three times now.)
A nearer to full tank keeps important components cool - fuel pumps live inside fuel tanks in modern fuel injected cars, this is important in hot weather. They can be expensive to replace.
Keeping the tank over half full also tends to prevent running out of gas, which may be the primary intent of the “old wives tale.”
When I got my first car my Great Grandfather told me so. He is ussually right about these things. Maybe his info was a bit dated. Oh and I did not see GorillaMan’s post when I typed that post.
I wonder if this wives tale is fuelled (heh) by the tendancy (amongst cars I’ve driven) for the fuel guages to be calibrated so that half is more like 1/3 and the 1st “1/2” of fuel appears to be used a lot more slowly than the second “1/2”. This would lead people to believe that they were getting worse economy on a low tank.
My father owns a gas station. The worst thing that can happen is when the fuel tank is drained. The fuel pump will get clogged by the crap in the bottom of the tank. What you say makes sense but experience says otherwise! 
maybe the bad stuff floats.
Hrm… I’ve kind of noticed that the first “half” of my full tank seems to burn a lot quicker than the last “half”. This may just be my mistaken non-scientific observation, but there you are.
I do wonder how accurate most gas gauges are, though.
Answer to OP. NO!
Fill the tank when the level gets down to 1/2 or 1/4 but keep it filled up to minimize the possibility of forgetting to fill up and running out at an unconvenient time and place. Make a habit of filling up on pay day, Friday, or some other regular time.
Know how much your tank holds per owner’s manual.
Take note of how many gallons are required to fill it to auto fill shut off point. The difference is appromimately how many gallons were actually in the tank at the 1/4 or 1/2 full gage reading.
Modern gas tanks shouldn’t corrode if your car isn’t ancient. Of course an ancient car negates the possibility that your gas tank is modern. In fact, the vast majority of fuel tanks on consumer vehicles these days are plastic! In the case of steel, your tank is painted inside and out, and it’s good stuff that won’t go anywhere. Condensation from a low tank shouldn’t be a problem.
Fuel pumps likewise don’t need cooling. They’re designed to work whether they’re submerged or not. How were pumps cooled before they were inside the fuel tank?
Finally, the fuel pump is usually in the tank at the point of a resevoir. This resevoir serves multiple purposes but the most important is this is where all the gasoline is drawn out of the tank. It’s usually the lowest point of an installed tank. Particulate matter which is heavier than gasoline will form here whether or not the tank is empty of full, and it will be caught by the filter. Okay, so there’s the danger of plugging your filter – but the fact is there isn’t a whole lot of particulate filter that gets into your tank. The most serious concern is the particulate that is generated as a result of the resistance weld that holds the two tanks halves together – the paint I mentioned is on the steel before it’s welded, so its vapor and residue stay within the tank. Newer paints especially help avoid this problem. In any case, particulate contamination is measured regularly at the plant to ensure this doesn’t become a problem.
So, unless you have a bad neighbor kid with some extra sand, or you keep your cars for 20 years, you shouldn’t have a whole lot to worry about.
The electric motor that actually powers the pump does need to be cooled by the gasoline, as it is not subject to any cooling airflow, since it is sealed inside the fuel tank. When fuel pumps were under the hood, they were mechanical (driven off the camshaft) and there was no electric motor to need cooled.
::: sigh::: It really is taking longer than we thought.
OK here is the deal. Assuming a modern fuel injected engine here is how the fuel system works.
[ul]
[li]The fuel pick up is at the bottom of the tank (it has to be because the pickup cannot pick up any fuel below the pickup.) [/li][li]The pickup usually has a sock type filter on the end of it (designed to keep leaves, dogs, and small children out of the pump)[/li][li]the fuel may then go to a pre-pump (a small pump designed to supply the main pump with a steady supply)[/li][li]Then on to the main pump (where the pressure of the fuel is bumped way up)[/li][li]After the main pump the fuel flows through the main filter.[/li][li]On to the engine[/li][li]There may be a return line back form the engine for any unburned fuel. Newer systems don’t use return lines.[/li][/ul]
The sock filter and pre-pump are always inside the tank. (if equipped)
The main pump, and main filter may in the tank, or they may be elsewhere, typically under the car. The newer the car the more likely it is that the pump and filter are inside the tank.
The amount of crap floating in the tank is a constant (assuming that you are not constantly adding more from crappy fuel). However ratio of fuel to crap obviously changes as the level of the fuel goes down. However unless you have a neighbor kid that throws dirt into your tank, this should not present a real world problem.
Now as far as water goes, don’t forget that any water that water is heavier than gas, so any condensation will go to the bottom of the tank. Don’t forget that the bottom of the tank has the pickup, so that any water blobs floating around will probably get sucked up and sent to the filter. The filter will act as a very rudimentary water trap. Meaning it will trap a few drops now and again, but if you have a quart of water in the tank, the filter will be overcome.
Note: some FI systems are very tolerant of water, other systems are very intolerant of water, and damage will occur if any water gets into the FI system past the filter. In either case if a slug of water gets past the filter the engine will run rough at best and most likely flat die. In all the years of working on cars the only water problems I have seen are from water pumped in when the tank was filled (Leaky gas station tanks) NOT condensation.
The fuel pump motor does get cooled by the fuel, but the level of the fuel is immaterial. Any fuel passing through the pump will do the cooling. If there is no fuel passing through the pump the engine will stop and so will the pump. (Electric pumps do not run with the engine off)
Oh yeah, tanks on modern cars might be plastic or they might be steel, depends on the design.
Gas gauges (as has been pointed out in several recent threads) are not precision measurement devices. My personal WAG based on all the years I have been around cars is that probably 45% of the gauges take forever to get off the full mark, and die the instant the needle hits E. Another 45% will have the needle fall off F about the time you drive out of the station, but can drive almost forever once the needle gets to E. Maybe 10% of the cars are fairly linear. YMMV.