when should you fill up?

ken_vs_ryu · #1 (**permalink**) 03-10-2009, 11:11 AM

should you wait until its almost empty to fill up the tank? does it matter?

E = Mc2 · #2 (**permalink**) 03-10-2009, 01:23 PM

Several schools of thought here. For maximum fuel economy, the tank should be filled only when absolutely necessary. The reason: As you use fuel, there is less weight in the car. Letting it get down to near nothing in the tank provides the opportunity for better overall FE.

Another school of thought says to never let the tank get below 1/2 full. The reason: As you use fuel, the tank fills with air [nature abhors a vacuum (*)] and allows more fumes to form in the tank. More fumes in the tank and more air in the tank equals a more explosive mixture should the tank be ruptured in an accident. I don't know how much this applies to our plastic tanks, but that is the reason given.

Personally, I don't think about filling up until the low fuel light comes on. In my case, I know that I have at least 50 miles worth of fuel when that happens. The only time I'll fill up before the light comes on is if I know I'm going to run out before I get out of Jefferson county (aka 'Metro' Louisville). I refuse to use Jefferson county's 'reformulated' gas, as it costs more, gets less mileage, and eats up fuel level senders.

(*) When I was in the Army, one of our HMMWV's (aka HumVee) was sent 'up the hill' to our support shop (3rd shop) for an engine change due to 'no power on acceleration'. Our shop wasn't allowed to change engines, btw, due to our job classification and the shop budget of $3000 a week for parts. The engine exchange would have blown our entire week's budget. On its return to our shop, the vehicle still had no power and was slated to be sent back to 3rd shop for another replacement engine. I asked if I could run the test regimen for that vehicle first. The first thing the book said to test was fuel pressure.

As it turns out, the HMMWV has a really long rubber tube attached to the fuel tank, with what turns out to be an old Jeep fuel filter on the end. The tube is to allow air to flow into the tank as fuel is pumped out. The filter is supposed to keep dirt out of the fuel tank. In this case, the filter was clogged solid with mud. Replaced the filter and had full power. What was happening was that the fuel pump couldn't overcome the vacuum formed in the tank and the engine wasn't getting enough fuel and fell flat when you stomped the accelerator. Someone got reamed over that one, as they had sent the truck up the hill for a $3500 engine when all that needed to be replaced was a 51 cent filter.

DeeZeex · #3 (**permalink**) 03-10-2009, 11:05 PM

Fuel in the tank helps cool the fuel pump. You increase the risk of pump failure by running the tank close to empty. Also, this morning was -39C, and at these temps, I always keep my tank above 3/4 full to prevent freeze ups and condensation/moisture build up in the tank.

E = Mc2 · #4 (**permalink**) 03-11-2009, 05:37 PM

^

That's the other reason given for maintaining fuel levels at or above 1/2 tank. A little alcohol used occasionally will help remove any water in the fuel tank, but the Canuckians probably already know that.

fittmann · #5 (**permalink**) 03-11-2009, 06:38 PM

I used to be one of those people that would fill up, drive to practically empty and fill up again. NOT ANYMORE...With the wild & crazy fluctuations in prices nowadays, I advise my friends (and especially my women friends) that ANYTIME you see gas as cheap as, or cheaper than your last purchase, if there is room in the tank, then top it off. Prices go up and stay up, much longer than they take to come down.I also attribute this to watching people freaking out about limited/no gas supplies after the past two hurricane seasons!!

'07 VBP Sport 5MT w/minor appearance & comfort mods!!

mahout · #6 (**permalink**) 03-11-2009, 08:28 PM

Quote:

Originally Posted by ken_vs_ryu

should you wait until its almost empty to fill up the tank? does it matter?

You should fill up anytime you don't have to go much out of your way to reach the gas station of your choice. Generally that also means less than half-tank but then if that's going to mean a big side trip not useful.

Sugarphreak · #7 (**permalink**) 03-11-2009, 11:03 PM

Quote:

Originally Posted by mahout

You should fill up anytime you don't have to go much out of your way to reach the gas station of your choice. Generally that also means less than half-tank but then if that's going to mean a big side trip not useful.

I agree, too much work trying to micro-manage something like this. As soon as my tank dips below half a tank I start keeping an eye out for gas stations, it never hurts to have some extra fuel in the tank. It has saved me a lot of hassle a few times in the past and lets me fill up at cheaper locations.

ToFit2Quit · #8 (**permalink**) 03-23-2009, 03:40 AM

When gas prices go down...fill the tank if I think prices are going to to up...when gas prices go up, buy just enough gas to get by for the week... and when it goes back down...buy just enough to get though the week until I think it's going to go back up...

I've seen my empty light go on only once so far and don't really care about the condensation micromanagement stuff because I just put concentrated fuel injector cleaners in every 3000 miles and before oil changes.

Black01 · #9 (**permalink**) 03-31-2009, 10:16 PM

I filled up a little early today, the needle was just a little below the half tank mark. I was at 265.1 miles then refilled at 5.5xx gallons, got a whooping 48mpg! This is the highest mpg that I got so far.

nosewitdot · #10 (**permalink**) 03-31-2009, 10:25 PM

wow...i dont understand how people get 40+mpg in their fits......it never works for me

Daemione · #11 (**permalink**) 03-31-2009, 10:53 PM

Try a beatrush underpanel . . . I put one on recently, and got 41mpg without even trying - manual transmission, ~70mph set on the cruise control. And that was with crummy/draggy winter tires, I can't wait to get a good highway trip in now that I've got the summers back on and the weather's warming up. For what it's worth, my previous best was 36mpg.

Type 100 · #12 (**permalink**) 04-01-2009, 07:14 AM

Quote:

Originally Posted by DeeZeex

Fuel in the tank helps cool the fuel pump. You increase the risk of pump failure by running the tank close to empty. Also, this morning was -39C, and at these temps, I always keep my tank above 3/4 full to prevent freeze ups and condensation/moisture build up in the tank.

One other reason I heard of discouraging the wait till the tank becomes bone-dry: Dirt in the tank.

The story goes like this:
Most gasoline stations store their fuel in large underground tanks or vats. If you gas up somewhere that's flood-prone (i.e. a lot of places in the Philippines...yeah, crappy drainage), there's a chance some of the flotsam in the floodwater can seep into these tanks. When you fill up at the said flood-prone gas station, that gunk can end up in your fuel tank, where it sinks to the bottom.

Once you let the tank run dry, you supposedly introduce the chance of the fuel pump ingesting this crud. I've heard stories from friends that wait till their tanks run low before filling up. When they took out their fuel filters for replacement, they were gummed up with mud and dirt - and offered the above story as explanation.

I'm not sure how believable this is...might this require a call to the Mythbusters?

===

In any case, personally I start thinking of fillups when my fuel gauge needle goes lower than 1/4 full. I rarely let it reach the point where the light comes on.

awptickes · #13 (**permalink**) 04-01-2009, 10:08 AM

In performance driving, and with airplanes, they calculate the exact amount of fuel that they will need for the journey, then fill that much. The idea here is that they've reduced the amount of weight. For cars, this doesn't work as well, as there's rarely traffic in a car-race, or in the air.

That said, I fill up when it gets to about 1/4 of a tank left.