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Old 01-22-2013, 08:52 AM   #3101
myrt1987
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Quote:
Originally Posted by CLL_OBP View Post
Oh, now that's something totally different. I haven't noticed anything like that and I fill up at half tank.
I mostly notice it with 1/4 tank or less it takes more gas to maintain the same speed. I cant explain it myself.
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Old 01-22-2013, 10:07 AM   #3102
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Originally Posted by myrt1987 View Post

I mostly notice it with 1/4 tank or less it takes more gas to maintain the same speed. I cant explain it myself.
I've noticed it too. On other cars as well. The first half of the tank gets you a lot more miles than the second. My dad explained why to me at one point. Can't remember. He also was adamant about not letting the gas tank get below 1/2 full & that was one of his reasons why.
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Old 01-22-2013, 10:18 AM   #3103
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An explanation for the mpg issue is your car has more momentum with a full tank. Aerodynamics matter less when your car weighs more and you are traveling at a constant speed. Js.
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Old 01-22-2013, 10:32 AM   #3104
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Quote:
Originally Posted by myrt1987 View Post

I mostly notice it with 1/4 tank or less it takes more gas to maintain the same speed. I cant explain it myself.
Just a wild guess but maybe it could have something to do with the smaller amount of gas and the EVAP system / charcoal vapor canister. The system was designed to reduce pollution and draw fuel vapor into the engine to increase fuel efficiency. As the fuel level decease, the concentration of vapors decrease and the mpg's decrease. Feels like I'm grasping at straws, lol.
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Old 01-22-2013, 10:34 AM   #3105
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Originally Posted by myrt1987 View Post
I mostly notice it with 1/4 tank or less it takes more gas to maintain the same speed. I cant explain it myself.
It's not that your car is consuming more gas when you get down to the last 1/4 tank. It's that the tank is tapered on the bottom, so the bottom portion of the tank holds less volume that the upper portions of the tank. It has nothing to do with momentum, weight, consumption of gas, and so on.

And the main reason many people fill their tanks at around the 50% point is to avoid the accumulation of condensation in the tank. This is most important in the winter time.
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Old 01-22-2013, 10:13 PM   #3106
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Originally Posted by goregasm View Post
An explanation for the mpg issue is your car has more momentum with a full tank. Aerodynamics matter less when your car weighs more and you are traveling at a constant speed. Js.
Just because the car has more momentum doesn't mean the aerodynamics matter less, except relatively with acceleration and hills. The additional weight will compress the springs a little more, but I can't see the 50 to 75 lbs making that much of a difference.

On an ideal flat and level road there will be no difference once the vehicle is up to speed, except rolling resistance, which is a minor consideration. Someone here had some stats on that - maybe the Commander.

I have noticed the dash gauge is a little more optimistic when the tank is fuller though - are you reading off the dash gauge or actually miles/gallons at a fillup?
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Old 01-23-2013, 03:21 AM   #3107
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Originally Posted by sgoldste01 View Post

It's not that your car is consuming more gas when you get down to the last 1/4 tank. It's that the tank is tapered on the bottom, so the bottom portion of the tank holds less volume that the upper portions of the tank. It has nothing to do with momentum, weight, consumption of gas, and so on.

And the main reason many people fill their tanks at around the 50% point is to avoid the accumulation of condensation in the tank. This is most important in the winter time.
Are you saying that a half full tank is not actually half full according to volume? But half way up the tank? That can't be right. Even given the shape & the sediment, etc. that's another topic & another reason to not wait until your tank is low to fill up. But that can't be the case. 1/2 full should be 1/2 by volume. How else would you define half?
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Old 01-23-2013, 04:26 AM   #3108
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By the time the fuel level gets low, some of the more volatile compounds in the fuel may have been boiled off and burned through the vapor system.

Additional weight hurts city driving more than highway, but it never helps.
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Old 01-23-2013, 06:36 AM   #3109
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Quote:
Originally Posted by SleepNMnky View Post

Are you saying that a half full tank is not actually half full according to volume? But half way up the tank? That can't be right. Even given the shape & the sediment, etc. that's another topic & another reason to not wait until your tank is low to fill up. But that can't be the case. 1/2 full should be 1/2 by volume. How else would you define half?
They just use a float to measure fuel don't they? So when it is at 1/2 you very well could have less fuel remaining than the first 1/2 due to the shape/volume
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Old 01-23-2013, 06:47 AM   #3110
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Originally Posted by SleepNMnky View Post
Are you saying that a half full tank is not actually half full according to volume? But half way up the tank?
Yes, that's exactly what I'm suggesting.
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Old 01-23-2013, 07:17 AM   #3111
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Well ludicrous as it sounds to not be measured by volume, I've learned something new. The tank is NOT half full. http://auto.howstuffworks.com/fuel-gauge1.htm

Last edited by SleepNMnky; 01-23-2013 at 07:23 AM.
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Old 01-23-2013, 07:19 AM   #3112
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Originally Posted by flyboy1100 View Post
They just use a float to measure fuel don't they? So when it is at 1/2 you very well could have less fuel remaining than the first 1/2 due to the shape/volume

That's easy enough to check - fill it up when it's half full. It would be easy to alter the float measurement to adjust for that - I would find it difficult to believe Subaru doesn't do that. How about the dash gauge? I haven't noticed a big change with my Scangauge.
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Old 01-23-2013, 11:07 AM   #3113
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The MPG numbers are not based on the float.
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Old 01-23-2013, 11:26 AM   #3114
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The MPG numbers are not based on the float.
Agreed. I'm simply explaining why the 2nd half of the tank seems to provide less range than the first half of the tank.
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Old 01-24-2013, 11:35 PM   #3115
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Agreed. I'm simply explaining why the 2nd half of the tank seems to provide less range than the first half of the tank.
Do you mean when the gauge reads half tank it doesn't fill up to half the spec'd volume or what? What are you basing this on? Fuel gauge, tank fill vs. mileage, dash gauge, Scangauge (or similar) or what?
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Old 01-25-2013, 06:23 AM   #3116
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Originally Posted by stevehnm View Post
Do you mean when the gauge reads half tank it doesn't fill up to half the spec'd volume or what? What are you basing this on? Fuel gauge, tank fill vs. mileage, dash gauge, Scangauge (or similar) or what?
On page 124 I described exactly what is still being questioned. (Which has been answered a couple of more times since.) A simple float is what has been used for many years. It used to be brass float soldered onto an steel arm. Nothing magic. It measures the level of fuel in the tank, not the volume. On older cars, tanks were typically square, rectangular, etc. Not complex shapes to fit around suspension or anything, so it was pretty consistant. That has changed over the years, but a float is still a float. Like I said before, if a pyramid was the shape of your tank the level would drop quickly near the top and slow down as you worked your way to the bottom, (volume). If the pyramid was upside down, the level would drop slowly at the top and much quicker toward the bottom. A float is a simple device. Like I said, don't confuse level and volume.

As for the dash puter, it does not calculate based on the float level. It uses the puter and bases its readings on fuel flow, extrapolating injector pulse, air temp, engine temp, rpm, etc. It is giving you numbers based on engine management parameters.

John
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Old 01-25-2013, 06:43 AM   #3117
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I'm getting awfully sick of always having a headwind. On this tank I have only driven 3 times.
1. Monday, -5f, 20mph headwind home.
2. Yesterday morning, -10f in the morning, 15-20mph headwind.
3. Last night, +10f, 28 mph headwind.

Now this morning when I leave it will be +1f, 35 mph headwind. Sigh.

The wind need to stop switching every 12 hours from north to south back to north etc
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Old 01-25-2013, 09:46 AM   #3118
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My gas mileage has sucked lately because it has been so cold! single digits or teens on my commute. I won't really be able to see if the reflash has helped until it warms up a bit. I do feel like the car is coasting better though, and also boggs down less, so I'm glad I had the reflash done.
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Old 01-25-2013, 12:38 PM   #3119
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Yeah, mine too. Looks like Flyboy is dealing with conditions worse than mine though.

John
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Old 01-25-2013, 01:39 PM   #3120
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Yeah, mine too. Looks like Flyboy is dealing with conditions worse than mine though.

John
I am expecting another 25mpg tank. The only thing I could control is the speed, but I can't wake up early enough so I do the limit or faster. Can't do anything about subzero temps, winter blend, and wind.

My Buick however in the same conditions is only getting 20mpg....
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Old 01-25-2013, 03:12 PM   #3121
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sooooo.....yea im just going to put this right here http://forums.nasioc.com/forums/show....php?t=2427586
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Old 01-25-2013, 07:53 PM   #3122
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sooooo.....yea im just going to put this right here http://forums.nasioc.com/forums/show....php?t=2427586
So I am not crazy.. thank god. Where do I sign up?
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Old 01-25-2013, 08:31 PM   #3123
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sooooo.....yea im just going to put this right here http://forums.nasioc.com/forums/show....php?t=2427586
I didn't realize the reimbursement was annually for the duration of ownership. I want to sign up now.
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Old 01-25-2013, 08:38 PM   #3124
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i could use some money. so i suppose small claim law suits? then move our way up like the honda hybrid civic lady. and then bam! so who wants to do the petition?
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Old 01-25-2013, 08:58 PM   #3125
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have to drive 90 minutes each way to a basketball tournament tomorrow with 3 other good size guys. curious to see how the wifes car will do carrying some beef!
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