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Old 08-13-2008, 08:23 AM   #1
spencerb
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Default Worse Gas Mileage Due to A/C or BC Error?

When I got the Astra in March, I would get around 34mpg on every tank. Now I'm getting 30 or 31.

I assumed it was due to increased A/C use (as others have reported here). But I noticed something else. When I got the car, I checked the gas pump readings against the BC (often I would go to the same pump), and the BC said I used a tad less gas (0.1 gallons or so), which meant that the BC showed an average mpg a little higher than actual. This was pretty consistant.

I didn't really check the gas mileage against the pump readings for awhile after that, just going by the BC.

Lately I've started checking. I started noticing that the BC says I use more gas than I actually do. Yesterday I went to the same pump as the previous fillup, and the pump said I used 10.4 gallons, and the BC said I used 11.2. Going by the pump, I got 33.3mpg, which is similar to when I got the car. BC said 30.9.

So.... is the A/C (or some other driving condition) really dropping my gas mileage by 3-4mpg, or is it false BC readings? What could be causing this? Has anyone else noticed discrepancies between the gas pump and BC?

...
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Old 08-13-2008, 09:21 AM   #2
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

The BC has a far better idea of how much fuel you used than the pump does. I'd go by that.

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Old 08-13-2008, 11:15 AM   #3
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

The BC shows how much fuel was used. It has no idea how much fuel was in the tank at the beginning of the trip. I'd say that the BC will give you the most accurate fuel use figures for the reasons I've listed below - basically the human/mechanical variables at the pump.

Depending on temperature and the vagaries of the pump you may get the same amount back in at fill, or you may not. Some of the variables include the nozzle shut off calibration, the ambient temperature at time of fill, flow rate of the nozzle. It also depends on whether or not you add fuel after the nozzle shuts off, and whether you hold the trigger full on for fastest possible fill or at the lowest flow rate to delay the blowback that triggers the nozzle shutoff.

The A/C will suck a couple miles per gallon easily. A local TV station did a test using a 2005 Ford Explorer. They ran the test on a dyno to simulate the driving and used a fuel flow meter similar to the BC in the Astra. They found that in city driving - stops & starts, it trashed the mileage by as much as 5-6 mpg. At highway speeds ~55-60, the hit was more like 2-3 mpg.

The fuel composition has a lot to do with mileage, too. Fuel blends change drastically based on locale, and season. Generally, winter/cold weather driving yields worse mileage because of extra time (and fuel flow) until the engine is up to temp. Summer fuel may have properties to meet environmental concerns that cause a rich condition (ethanol added?)

Fuel economy is probably the hottest topic, and the one that has the greatest number of variables. Remember, "your mileage may vary".

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Last edited by SaturnRaycer; 08-13-2008 at 11:21 AM..

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Old 08-13-2008, 12:39 PM   #4
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

I take it you reset the BC at every fueling? Also it is just an indication, real consumption you have to calculate yourself. The BC consistently lies to me, it's more of a gizmo than something useful.

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Old 08-13-2008, 12:50 PM   #5
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by K-1 View Post
The BC has a far better idea of how much fuel you used than the pump does. I'd go by that.
I disagree. The difference you are quoting amount to about 7.8%. If you are putting in 12 gallons, this means you have to off by almost 1 full gallon.

Track the mileage over a couple of consective tankfuls to reduce the fill-up error. I'm guessing the BC is off.

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Old 08-13-2008, 01:41 PM   #6
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

my opinion on this is that when you fill up your car it isnt the same all of the time probably because of evaporative things and pressure. in the olden days you could fill right to the cap but now if you fill it once the next time you fill it isnt the same fill. so to be accurate you need to reset the computer with the next fill and keep track without resetting anything over 4 or 5 tankfulls. keep track of the total amount you put in over time anmd the total mileage over time not just one tankful and it will be more accurate

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Old 08-13-2008, 01:46 PM   #7
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

I reset BC 1 when I bought the car and pulled out of the lot. After 12K+ miles, one thing that's interesting is that the mileage is off a little. I don't remember the numbers off hand, but I think it shows about 0.5% more miles than the odometer (accounting for the 291 miles already on the car when I bought it).

I know there can be errors in the pumps. But here's the greater issue: The BC used to give me readings close to various pumps, but now it is off for the last few tanks I've checked.

I usually do reset BC 2 at every fillup. I'll let it run for a few tanks, total up the gas used per the pump, and try to average out pump error.

...
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Old 08-13-2008, 01:52 PM   #8
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by OPEL Blitz View Post
Also it is just an indication, real consumption you have to calculate yourself.
Yes and yes, but.

Yes. The fuel consumption values on the BC along with inst economy and o/a economy are calculated by the BCM from the injector duration. Since the computer has been "taught" the flow rate of the injectors, it can calculate the consumption based on the amount of time the injector is open. This is controlled by load, temp, and feedback from the O2 sensors.

Yes, but. The weakness in the consumption calculation is achieving consistency. As mentioned in my previous post, there are human variables, as well as variables in the pump - mostly at the nozzle. You can mitigate these by following a routine - use the same pump at the same station at the same time of day. This is rendered moot when the station does maintenance on the pump, changing the nozzle out - most nozzles have a life span of < 1 year before being changed because they fail.

Temperature plays a role in this too. The pump is calibrated with "corrected to" temperature for delivery. Your fuel tank is not. Additionally, gasoline becomes more dense as the temperature drops. You may be able to put 12.0 gallons in a completely drained tank when the temp is 32f, but only 11.8 gallons on a 75f day.

As others have said, to get the most accurate consumption figures, you have to average it over a number of tankfuls. Whether you calculate every 4 tanks, or every 1000 miles, doesn't matter. Just as in statistics, the larger the sample size, the more accurate the measurement. I keep a running calculation in a spreadsheet. The formula is: current miles-miles at vehicle delivery/sum fuel consumed.

Using this logic my last tank was 32.462; last 1k miles (7701-6715) 32.87; and avg mileage since delivery 30.702.

...
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Last edited by SaturnRaycer; 08-13-2008 at 02:04 PM..

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Old 08-13-2008, 03:40 PM   #9
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by SaturnRaycer View Post
Temperature plays a role in this too. The pump is calibrated with "corrected to" temperature for delivery. Your fuel tank is not. Additionally, gasoline becomes more dense as the temperature drops. You may be able to put 12.0 gallons in a completely drained tank when the temp is 32f, but only 11.8 gallons on a 75f day.
I'm no chemist, but i've never really understood something about this. If the tanks at the gas station are underground, shouldn't the gas be a fairly consistent temperature? Most of the time after I get gas, if I touch the part of the handle where the gas flows through, it's much colder than before I operate the pump.

...
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Old 08-14-2008, 10:54 PM   #10
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by L200Pilot View Post
I'm no chemist, but i've never really understood something about this. If the tanks at the gas station are underground, shouldn't the gas be a fairly consistent temperature? Most of the time after I get gas, if I touch the part of the handle where the gas flows through, it's much colder than before I operate the pump.
They are underground, and yes the temperature is theoretically more stable there, but they aren't all that far underground. They have to be below the frost line - in most cases 4 feet at the top. Not far enough to be exempt from temp changes due to ambient ground level temps. Depending on location (Phoenix or Tucson) you can have a measurable difference in density. Another reason not to "top off" the tank.

If you get far enough below ground - say 50 -100 feet the temps stabilize at about 50f year round. This is one of the principles of using the earth to help with air conditioning a building.

The fuel is cooler than the ambient temp in summer, and warmer in winter, hence the "cool hand" on the pump handle.

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Old 08-15-2008, 06:26 PM   #11
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Last night on the flat I-10 cruising at 70 mph that my mileage was 30 mpg with the AC off, and 29.5 mpg with the AC on.

I was using the ScanGaugeII which is pretty spot on because I enter the exact fuel used with every fill up.

I assume a greater mpg loss on the Astra because of it's smaller engine.

...
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Old 08-21-2008, 03:18 PM   #12
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Not a mileage question but a BC error...
I have noticed that when the BC says Refuel!!! Range ---, and I fill up I'm still only able to get approx 40 litres of fuel in the tank. Owners manual states that its a 60 litre tank so by my math its still 1/3 full when the BC shows Empty!
has anyone else noticed this or have the same problem?

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Old 08-21-2008, 03:42 PM   #13
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by TortoiseMe View Post
Not a mileage question but a BC error...
I have noticed that when the BC says Refuel!!! Range ---, and I fill up I'm still only able to get approx 40 litres of fuel in the tank. Owners manual states that its a 60 litre tank so by my math its still 1/3 full when the BC shows Empty!
has anyone else noticed this or have the same problem?
Here on the gm canada website, they say that the fuel tank is 45.5L
Look at the end of the page :
http://www.gm.ca/ss/gm/options.do?mo...layMode=normal

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Old 08-21-2008, 03:56 PM   #14
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by TortoiseMe View Post
Not a mileage question but a BC error...
I have noticed that when the BC says Refuel!!! Range ---, and I fill up I'm still only able to get approx 40 litres of fuel in the tank. Owners manual states that its a 60 litre tank so by my math its still 1/3 full when the BC shows Empty!
has anyone else noticed this or have the same problem?
I would say you might have a BC problem Mate. When mine says !!!Refuel!!! it means it.

The US capacity is just under 12 gallons, don't have the exact figure with me. The one time I took it down to the !!!REFILL!!! I put in just over 11 gallons.

I watch my gas mileage pretty close using my trip computer and the estimated range on the BC. So far it has been pretty accurate.

I have found the BC estimated MPG to be just a little lower then if I calculate it myself. Usually off by a few tenths but never more then 1 MPG.

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Old 08-21-2008, 04:33 PM   #15
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

I wonder why they fitted a smaller tank, 45l vs 52l.

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Old 08-21-2008, 04:43 PM   #16
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by OPEL Blitz View Post
I wonder why they fitted a smaller tank, 45l vs 52l.
Most likely US regs required some safety equipment installed which may have taken up some room. Hence the smaller tank.

When the GTO was imported from Australia, we went thru a similar scenario.

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Old 08-22-2008, 10:11 AM   #17
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

u can now import a used gto from the u.s. into canada. cdn bumper standards are now harmonized with the u.s.

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Old 08-26-2008, 11:23 AM   #18
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

Quote:
Originally Posted by SaturnRaycer View Post
Additionally, gasoline becomes more dense as the temperature drops. You may be able to put 12.0 gallons in a completely drained tank when the temp is 32f, but only 11.8 gallons on a 75f day.

Um...NO! The volume the tank holds is the same regardless of temps. The tank would still hold 12 gallons regardless of temps. The difference is simply that 12 cold gallons is denser that 12 hot gallons.

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Old 08-27-2008, 05:56 PM   #19
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Default Re: Worse Gas Mileage Due to A/C or BC Error?

UPDATE:

I've made 3 fillups. Unfortunately, I wanted to use the same pump I started with to get a more accurate shutoff point, but the station is closed for remodeling.

Each fillup was at a different station.

BC readings:
33.2 gallons used
1032.6 miles
31.1 mpg

Gas usage per pumps: 29.8 gallons
Mpg per pump usage and BC mileage: 34.7

This gas usage error with these three tanks was 11.4%, and the error with my earlier single-tank fillup (same pump both times) was 7.7%. When I got the car the error was around -1% (when I got the car, the BC showed better than actual mileage, now it shows worse than actual).

...
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