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Old 01-02-2013, 12:42 PM   #1
delmarco
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Question Possible Easter Egg Concerning Gas Tank/Fuel Gauge?

Since the NY Tri State Gas Shortage after Hurricane Sandy in November I have been keeping my tank full and refilling every time the needle gets to 60% (or a little above the halfway mark).

Before, since I do not drive my car daily, my general practice was to fill the tank to 50% full and then run the tank to 0%* before refilling to 50% when the low gas lamp came on.

However, now I'm finding that by keeping the tank full at 100% and refilling it back to the top from 60% that my MPGs are much improved and it takes much longer to go from a full tank to %60 than it does from 50% to 0%*.

It is almost like the engine is sipping gas from 100% to 50% then guzzling the rest of the gas as the tank empties from 50% to 0%.

Does the E46 fuel system optimize something in the performance when the gas tank is full compared to when it is at 50%?


*(by 0% I mean when the lamp comes on and not when the tank is actually empty)
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Old 01-02-2013, 01:00 PM   #2
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There is a certain amoung of optical dillusion going on.

You always had the 1st 1/4 tank last 150 miles and the last 3/4 tank lasting the other 150 miles. The "problem" is that the tank is smaller at the bottom than at the top, but the fuel level sending units are linear. The sending units do not adjust for the variance in size between the top and the bottom of the tank, so the top 1/3 of the tank includes the space over the top of the transmission hump, while the lower 2/3s are measurements taken below the hump.

The mileage should remain the same, but the rate of needle movement is different.

PS
Your car prefers you keep the tank full and then top off at a half tank as opposed to keeping the tank empty and filling it half way. The reason is, fuel acts as a heat sink for the fuel pump. If there is no fuel then there is no heat sink. The life of the fuel pump is shortened.
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Old 01-02-2013, 01:53 PM   #3
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I've had the same observation as delmarco and thought about it.. And I'm talking about measured mpg, not OBC-calculated.

I think as the tank gets less full, there's more air volume available for gas to vaporize. Once it's vaporized, it gets into the vapor canister and is never used at full efficiency. Keep the tank full, and you lose less gas through vaporization because of the increased vapor pressure. And get slightly better mpg..
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Old 01-02-2013, 01:59 PM   #4
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You can't judge MPG by how fast the gauge moves. My Jeep Grand Cherokee would do 250 miles on the right half and 150 on the left. The MPG was the same, the gauge was just highly non-linear. Our gauges are more accurate than that, but your car isn't using more gas when it's lower on fuel. The vapor thing isn't a problem either. Our systems are sealed so vapor does not escape and it gets used. That's part of the emissions standard our cars meet.
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Old 01-02-2013, 02:21 PM   #5
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My F150 goes 250-ish miles on a tank of gas, 100 on the first 1/4 tank, and 150 on the rest.
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Old 01-02-2013, 02:36 PM   #6
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My Neon R/T does the same.

150km on the first 1/4.
250km on the rest 3/4.

Has to be tank design, as mentioned above.
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Old 01-02-2013, 03:00 PM   #7
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I never thought about why it seemed non-linear to me...makes perfect sense.

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Old 01-02-2013, 03:42 PM   #8
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Quote:
Originally Posted by sammk View Post
I've had the same observation as delmarco and thought about it.. And I'm talking about measured mpg, not OBC-calculated.

I think as the tank gets less full, there's more air volume available for gas to vaporize. Once it's vaporized, it gets into the vapor canister and is never used at full efficiency. Keep the tank full, and you lose less gas through vaporization because of the increased vapor pressure. And get slightly better mpg..
Fill the tank.
Reset the odometer.
Drive.
Fill the tank.
Divide the distance travelled by the gallons put in.

The result is the miles per gallon. It should not matter that you go 150 miles on 7.5 gallons of gas or 300 miles on 15 gallons. You still went 20 miles per gallon. No matter how fast the gas gauge falls, you still get the same mileage.
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Old 01-02-2013, 03:45 PM   #9
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Quote:
Originally Posted by jdstrickland View Post
Fill the tank.
Reset the odometer.
Drive.
Fill the tank.
Divide the distance travelled by the gallons put in.

The result is the miles per gallon. It should not matter that you go 150 miles on 7.5 gallons of gas or 300 miles on 15 gallons. You still went 20 miles per gallon. No matter how fast the gas gauge falls, you still get the same mileage.
AND, the OBC calculations in my car are within a digit or two of the mileage I get with my calculator. Quite frankly, I can do more to have an error in what the car says I get and what my calculator says I get just by changing how agressively I fill the tank. Bottom line, if the car says you get 24.5mpg, then you are getting pretty damn close to 24.5mpg. That's been my experience, anyway.
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Old 01-02-2013, 04:20 PM   #10
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I've noticed the same thing. I think it's just the way the tank is designed.
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Old 01-02-2013, 07:11 PM   #11
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Quote:
Originally Posted by jdstrickland View Post
Fill the tank.
Reset the odometer.
Drive.
Fill the tank.
Divide the distance travelled by the gallons put in.

The result is the miles per gallon. It should not matter that you go 150 miles on 7.5 gallons of gas or 300 miles on 15 gallons. You still went 20 miles per gallon. No matter how fast the gas gauge falls, you still get the same mileage.
You're preaching to the choir. Note what I said above - measured mpg. FYI, I keep a spreadsheet where I track every single fill on both my E92 and E46. I know my real world mileage to the second decimal.

I do know the gages always have a non-linearity (which is much, much improved in the E9x BTW), but I thought I made it clear that's not what I'm talking about..

Quote:
Originally Posted by jdstrickland View Post
AND, the OBC calculations in my car are within a digit or two of the mileage I get with my calculator. Quite frankly, I can do more to have an error in what the car says I get and what my calculator says I get just by changing how agressively I fill the tank. Bottom line, if the car says you get 24.5mpg, then you are getting pretty damn close to 24.5mpg. That's been my experience, anyway.
That's not been my experience with the OBC calculator. The OBC always overpredicts by 1-2.5 mpg. And I can supplement it with real data and charts from my cars.
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Old 01-04-2013, 11:59 AM   #12
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Thanks guys. Great information. Really cleared it up for me.

I forgot to think about the 'shape' of the tank. lol.


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Old 01-04-2013, 12:25 PM   #13
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Quote:
Originally Posted by jdstrickland View Post
AND, the OBC calculations in my car are within a digit or two of the mileage I get with my calculator. Quite frankly, I can do more to have an error in what the car says I get and what my calculator says I get just by changing how agressively I fill the tank. Bottom line, if the car says you get 24.5mpg, then you are getting pretty damn close to 24.5mpg. That's been my experience, anyway.
Same here. Both of my E46's OBCs have been almost spot on. Generally .1-.2 off at most.
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Old 01-04-2013, 04:44 PM   #14
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See tank shape above. center of volume is definitely not halfway up. technically, you will get slightly worse mileage with a full tank due to weight, but i would definitely go out on a limb and say it is negligible. The biggest factors by far are maintenance and driving style, after that i would say probably filling up the whole tank completely then filling up when under a quarter, would be the optimized solution if you are looking to maximize efficiency. less trips to the gas station is a good thing. to maximize fun and character... well, that's another story.
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