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Old 24 December 2009, 04:41 PM
purpleiguana purpleiguana is offline
 
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Default Last 1/4 tank gets worse mileage than first 1/4?

From the start of my tank to the fumes, I can get a little over 350 miles (tank holds approx. 13 gallons, not that that's terribly relevant). Thing is, I've noticed that I can get over 100 miles from a freshly filled tank to before it hits the 3/4 mark. Assuming everything burned steadily, you'd think I could get over 400 miles per tank. I mentioned this to someone (I forget who--I think a guy I knew in college) who said it was common knowledge that the first quarter of the tank got the best gas mileage and to never let your tank get below 1/2 full if you wanted to maximize your mileage. (??)

I also heard somewhere--many, many, MANY moons ago--that if you're running on fumes and need to make what's left of your gas hold out for just a little bit longer, you should put your car in neutral when stopped at a stop light. I don't remember what logic (if any) was attributed to it, except that it was presumably less strain on the engine.

Is there any truth to either of these?
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  #2  
Old 24 December 2009, 04:44 PM
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snopes snopes is offline
 
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Ponder

I think the likely explanation is that since gas tanks are not uniformly shaped, the gauges aren't strictly accurate. When the gauge reads 3/4 full, you may have actually used a bit more than 1/4 of the tank, and when the tank reads 1/4 full, you may actually have a bit less than 1/4 tank left.
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Old 24 December 2009, 04:53 PM
Singing in the Drizzle Singing in the Drizzle is offline
 
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I agree. Gas gauges measure the depth of fuel in the tank, not the amount. My Jimmy drops to just above half full after 50 miles. Then slowly drop to the 1/4 full mark over the next 200 miles, were upon it quickly moves to empty. My other cars do similar things the gauge, just not as noticeably as the Jimmy.
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  #4  
Old 24 December 2009, 05:34 PM
KirkMcD KirkMcD is offline
 
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Jolly Roger

Have you ever ran your car until it really ran out of gas? You might get the miles you think you are missing.
Now a days, when you first hit E on the guage you usually still have a few gallons left in the tank.
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Old 24 December 2009, 05:37 PM
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Archie2K Archie2K is offline
 
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My car seems to get fewer miles in its last half than its first half. I just assume that the gauge doesn't go down evenly as suggested by snopes. However, I also think the effect may be accentuated by the fact that the fuel warning light comes on and the tank shows as empty when there is still perhaps 30 or 40 miles of driving left.

(Since it is an electronic guage I can tell exactly when it says 1/2 and when it says 0.)

In actual fact, I'd expect the opposite. Fuel is heavy so emptying a tank that had 20L should surely get better mileage than using the first 20L or a 40L tank.

ETA: First spanking in a long while!
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  #6  
Old 24 December 2009, 06:06 PM
noysey
 
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In the last two vehicles I've owned I would fill them up and the gas gauge would dwell on full for what seemed to be many miles before it started going down. However, I would always fill it when it read half empty, and the fill up volume would be about half a tank, based on it's size.

Getting away from gas gauges for a moment, maybe it is an UL but I remember hearing that using the last 1/4 tank means that you are sloshing the gas around in the tank as you drive and mixing any impurities that are normally settled on the bottom in with the gas. That might create the worse mileage you ask about.

Just in case, it is one of the main reasons I fill up when the gauge reads half full. One never knows...
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  #7  
Old 24 December 2009, 06:39 PM
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Daphodil Daphodil is offline
 
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One reason the gauge could vary is because a tank that is one-quarter full has more empty room for the fuel to slosh about versus a full tank. If the gauge is reading depth, the depth may vary a lot more as the car moves when the fuel level is lower.
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  #8  
Old 24 December 2009, 06:44 PM
Mycroft Mycroft is offline
 
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Quote:
Originally Posted by Archie2K View Post
In actual fact, I'd expect the opposite. Fuel is heavy so emptying a tank that had 20L should surely get better mileage than using the first 20L or a 40L tank.
I disagree; the weight of fuel in a car is negligible - 20 litres of petrol weighs less than 20 kg (petrol is less dense than water); wheras even a fairly small car (Ford Fiesta) weighs over 1500 kg.
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  #9  
Old 24 December 2009, 06:53 PM
purpleiguana purpleiguana is offline
 
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Quote:
Originally Posted by snopes View Post
I think the likely explanation is that since gas tanks are not uniformly shaped, the gauges aren't strictly accurate. When the gauge reads 3/4 full, you may have actually used a bit more than 1/4 of the tank, and when the tank reads 1/4 full, you may actually have a bit less than 1/4 tank left.
Now, see, that makes more sense. And I tend to find that things that make sense are generally true.

Does anyone have any insight about the whole idling at neutral thing?
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  #10  
Old 24 December 2009, 07:12 PM
Singing in the Drizzle Singing in the Drizzle is offline
 
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Originally Posted by purpleiguana View Post
Does anyone have any insight about the whole idling at neutral thing?
Assuming you are talking about automatic transmission. I should use less fuel since the engine should be doing lest work. I do not have the time to try and explain the working of an automatic transmission and fluid mechanics at this time.

Its off the see the family for Christmas Eve.
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  #11  
Old 24 December 2009, 07:38 PM
jimmy101_again jimmy101_again is offline
 
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Most modern automatic transmission cars will boost the idle speed a bit when idling in "gear". (The same thing will happen if you turn on the AC.) They have to because the trany adds a fair amount of load to the engine and if the idle speed isn't boosted a bit the engine may stall.

Faster idle speed means the engine is burning more fuel per minute. But the difference in fuel consumption is going to be so miniscule as to be irrelevant. Depending on the car, how long you idle etc., you are probably talking about changing how far the car will go before running out of gas by tens of feet.

On a modern car, turning the engine off at a stop light will save more fuel than will shifting to neutral. (Though it is still not all that much.) On older cars turning off the engine often ends up using more fuel because old carburetors used a big shot of fuel when the engine was started.

As to the accuracy of the fuel gauge; fuel gauges are notoriously inaccurate. Besides the possibility that the tank doesn't have a uniform cross-section, the sensor generally can't read correctly when the tank is near full or near empty since the float hits either the top or the bottom of the tank. Some cars can't read either correctly, some can read one but not the other. In addition, most fuel sensors are, because of their design, non-linear devices.

Sloshing of fuel in the tank rarely makes much difference in the reading. The circuitry is heavily "dampened", which means the reading is the average over a fairly long period of time, like a couple minutes. (You can estimate how heavily the gauge reading is dampened by noticing how long the gauge takes to read "Full" after filling up.)

The weight of the fuel in the tank does indeed affect the mileage. Probably not enough for you to actually notice the difference between a full and a nearly empty tank but enough to make a difference to the manufacturer. 70 pounds or so (about the weight of 12-ish gallons of gas) is significant enough. That is part of the reason why modern cars tend to have somewhat smaller tanks then the cars of thirty years ago. Weight savings is also a big part of why car makers went to the "limited use" spares, it saves a few pounds. So yes, even a few pounds can make a difference, though the difference it generally very small. (To a car manufacture for the US market boosting the mileage estimate by even 0.1 MPG is significant and represents a large amount of money to the manufacturer.)
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  #12  
Old 27 December 2009, 05:58 AM
rswarrior
 
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Every GM car or truck I've owned has had the full for a long time, empty last 1/4 real fast type of tank. My Hyundai Genesis Coupe and Dodge Stealth Turbo both show the fuel level dropping withing a few miles. It seems to me to be an indication that GM has a common issue with the fuel indicators not being terribly accurate when full. At least with it dropping fast on the last 1/4, it scared me into getting to a gas station faster.
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  #13  
Old 01 January 2010, 10:54 PM
Ulkomaalainen Ulkomaalainen is offline
 
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I remember my old Polo of 1988, which had the opposite effect: it went down to 1/2 full quite fast, but slowed down later on - most likely because it wasn't measured correctly. Since I had a normal driving routine, though, the kilometers driven were a way more accurate account of how much was approximately left in the tank (550: alert, 600: really time to fill up).

My "new" Polo unfortunately goes fast on the first half and even faster on the second... :-/
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  #14  
Old 02 January 2010, 03:32 PM
RichardM RichardM is offline
 
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As a general rule, manufacturers for the US market design their vehicles to go 300 miles per tank of fuel. They also design the fuel gages to be non-linear such that at 200 miles of travel, the gage shows half full. Since most people will fill up at half or slightly below, people than note that they went 200 miles on half a tank and therefore brag that they can go 400 miles on a full tank. This give a false sense of economy and may increas sales of that brand.

Very few of the stock gages on vehicles are linear or accurate.
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  #15  
Old 02 January 2010, 10:03 PM
MariaVerde MariaVerde is offline
 
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Quote:
Originally Posted by noysey View Post
Getting away from gas gauges for a moment, maybe it is an UL but I remember hearing that using the last 1/4 tank means that you are sloshing the gas around in the tank as you drive and mixing any impurities that are normally settled on the bottom in with the gas. That might create the worse mileage you ask about.
I'm pretty sure this was debunked on Car Talk a few years ago. It sticks in my mind because years ago, my boyfriend at the time told me I shouldn't drive around on E because I was ruining my engine. I didn't believe him, and thought the originator of the myth probably lived with someone who'd drive the family car down to the fume level and then hand the keys to the next person who needed it, forcing them to fill the tank.
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Old 03 January 2010, 01:38 AM
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I'd be interested in seeing this debunked, DH goes nuts if I let it get below half, because he claims that condensation forms in the empty space, and mixes with the fuel ruining the engine.

As to the sloshing, and the level on the gauge, I remember my Honda Civic's gauge would dip markedly if I was taking a steep right turn, around a cloverleaf, or similar, and actually go UP if I were taking a steep left.
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  #17  
Old 03 January 2010, 01:51 AM
robertplattbell robertplattbell is offline
 
 
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Default Some thoughts.

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Originally Posted by mouse goddess View Post
I'd be interested in seeing this debunked, DH goes nuts if I let it get below half, because he claims that condensation forms in the empty space, and mixes with the fuel ruining the engine.

As to the sloshing, and the level on the gauge, I remember my Honda Civic's gauge would dip markedly if I was taking a steep right turn, around a cloverleaf, or similar, and actually go UP if I were taking a steep left.
Several things:

1. Some older fuel senders used a float on an arm. Doing the geometry (of an arc) you'll see that the linear level changes non-linearly when transferred to a tangent arc. In the middle of the arc, the fuel level may be nearly 1:1. But at the top or bottom, it varies. Some fuel senders I have seen are nearly 1:1 at the top, and then vary, due to the arc-curve of the float arm, as the level decreases.

2. In the old days of vented gas tanks, condensation was a real problem. Keeping the tank full resulted in less water in the tank and less corrosion (or frozen gas lines in the winter). Since about 1973, gas tanks have been sealed and a charcoal canister (Evaporative Emissions System) or equivalent, is used to condense and burn the gasoline vapors. Evaporating gasoline, it turns out, was a major component of smog.

Today, with sealed gas tanks, I think the condensation thing is perhaps overstated. But I try to keep my cars filled, and when I put them in storage, I fill the tanks and put in the requisite amount of STA-BIL in the tank. Gas tanks today are more likely to be made of polypropylene or other non-corrosive materials as well.

3. The detritus in the tank theory is probably sound, but if you change your fuel filter often enough, and don't drive in Mexico (don't ask) it should not be an issue. Most US gas is pretty clean.

This reminds me of the U/L about the quarter dropped into the gas tank causing "mysterious" car problems. Do you remember that one?
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  #18  
Old 03 January 2010, 01:59 AM
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Eddylizard Eddylizard is offline
 
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The last few cars I've owned have advised in the handbook not to let the tank run to empty, the explanation being given was that gunk accumulates and gets sucked into the fuel system, blocking the fuel filter.

Whether that's sound advice or CYA by the manufacturers for a "get out of warranty free card" I don't know.

As for the OP, I've never owned a car with a fuel guage I could trust.
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  #19  
Old 03 January 2010, 02:05 AM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by mouse goddess View Post
I'd be interested in seeing this debunked, DH goes nuts if I let it get below half, because he claims that condensation forms in the empty space, and mixes with the fuel ruining the engine.
You might ask him how the engine manages to survive the water it creates when it burns gas.

An internal combustion engine generates a heck of a lot of water. Lets see, one gallon of gas, figure it's pure octane and weighs 6 pounds/gallon, 2C8H18 + 25O2 -> 16CO2 + 18H2O, 6 pounds of gas gives 18/2*18/114= 1.4 pounds of water. In volumes, 1 gallon of gas creates about 1.5 pints of water. (YMMV ;~ )

Water in the gas won't hurt anything. In an extreme case it might keep the engine from starting or make it run rough, or given just the right circumstances freeze in the fuel lines. It would take a fair amount of water to actually do any of those things.

If a person is really worried about water in the gas then they should just run a tank of ethanol containing gas through the vehicle every one in a while. The ethanol will tend to flush any water in the gas tank out and through the engine (which won't bother the engine at all).

There are some (wingnuts mostly) that believe injecting water into the gas improves engine performance. It doesn't. But it really doesn't hurt all that much either.
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  #20  
Old 03 January 2010, 01:45 PM
RichardM RichardM is offline
 
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Quote:
Originally Posted by Eddylizard View Post
The last few cars I've owned have advised in the handbook not to let the tank run to empty, the explanation being given was that gunk accumulates and gets sucked into the fuel system, blocking the fuel filter.

Whether that's sound advice or CYA by the manufacturers for a "get out of warranty free card" I don't know.

As for the OP, I've never owned a car with a fuel guage I could trust.
Check that owner's manual again. At least one of the cars I've owned, the manual said to not let the fuel level get too low because the fuel pump was in the tank and was cooled by the fuel. Let the level get too low and the pump would get hot and fail.
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