Accuracy of Gas station pump

[benfrogg]

So, in recent months I've been noticing that my MPGUINO display has been getting farther away from my actual. It started about 2 mpg lower than the display said. For instance, if it says 64mpg, actual would be 62. The last several tanks, it's been going down. Each tank it seems worse when I still get the display to read 64-65mpg. This latest tank the reading was 58.82 whereas the display read 64.11.

What can be to blame here?
New gas tank, 4 months or so ago. No leaks. No fuel odor. Fuel cap holds pressure (gasket in good shape, hisses when opened after the tank has been depleted). No change in driving habits. No change in type of driving.

Temps have changed, but not significantly. It was averaging the 20s last month, it is averaging the 30's now, sometimes 40.

I have used the same pump at the same station for several months. Only thing I can think is that the pump is off. The theoretical reduction in actual fuel dispersed coincides directly with the price of fuel going up..... Mobil station......

I know thermal expansion is a possibility, but I'd assume that would work out over time.
Ideas?
B

[GasSavers_WD40]

Mines off by as much as 10% lately .. I was wondering how to adjust the MPGUINO by %

[benfrogg]

To adjust the display you simply take the usec/gal number and subtract 10% from it. More accurately, you could figure out the exact percentage you are off and subtract that percentage instead.

My quibble here is not that the gauge is off. The issue is that it was consistently 2 mpg low and now it is 5 or more low. That seems to me like a 5% difference or so.
I'm still thinking gas station. I'm going to try the place across town that is 24 hours (need a 24 hour station for my lifestyle)
I'm also going to get 1 gallon exactly from the pump I've been using. Then I'll take it home and measure it with a measuring cup to determine if it's really a full gallon or not. If not, I'll be on the horn with weights and measures.
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[FrugalFloyd]

I'd suspect something mechanical, rather than the gas station pump. When did you last check your spark plug gap, check for dragging brakes, clogged air filter, tire inflation pressure, etc?

[VetteOwner]

yea i blame the weather, was 60* last week now expecting snow showers and a high of 38....

[benfrogg]

The problem isn't lower fuel economy. The problem is the gauge is still telling me the same numbers whereas the actual is down.
The MPGUINO uses an injector lead, divided by the transmission output sensor to calculate mpgs. Thereby, if an outside source was lowering my FE (such as dragging brakes, plug gap, etc) I'd be using more throttle/ the injectors would be open for a longer duration. If that were the case, the MPGUINO would be lower as well as the actual.
When you program the MPGUINO, you have to enter data that tells the computer what the amount of flow is. It's listed as USEC/GAL.
So, the only way that these FE numbers could be changing is if A) the injectors are somehow flowing 5% more fuel per pulse; B) There is something wrong with the MPGUINO; C) Fuel leak; D) Thermal Expansion; E) Gas station inaccuracy.
A seems unlikely, but probable if there was some piece of crap in one of them?
B possible
C unlikely given the circumstances
D probably not helping, but 5%???
E most likely that I can see

Thanks for the input so far!
B

[benfrogg]

VetteOwner-
Where do you live?
I usually fill up at 2 in the morning; it's pretty cold then every night. The temps have been consistent during fill ups. (+ or - 7degrees)
B

[FrugalFloyd]

OK, if we assume the fuel injectors are working the same, why wouldn't a too-wide spark plug gap show the same mpg, while the decreased combustion efficiency causes real fuel economy to suffer? Likewise, if tire pressures drop, that wouldn't be reflected in any change in fuel injector pulse length, would it?

[theholycow]

Originally Posted by benfrogg

So, the only way that these FE numbers could be changing is if A) the injectors are somehow flowing 5% more fuel per pulse; B) There is something wrong with the MPGUINO; C) Fuel leak; D) Thermal Expansion; E) Gas station inaccuracy.

Check your fuel pressure. If your pressure regulator is out of whack then that could do it.

Originally Posted by SentraSE-R

OK, if we assume the fuel injectors are working the same, why wouldn't a too-wide spark plug gap show the same mpg, while the decreased combustion efficiency causes real fuel economy to suffer? Likewise, if tire pressures drop, that wouldn't be reflected in any change in fuel injector pulse length, would it?

Changes in efficiency would show up in fuel usage. Accurate measurement of fuel usage would show up on the device. If it couldn't measure the difference in fuel economy from tire pressure or combustion efficiency, then what difference COULD it measure? It would be totally worthless, might as well just write a number on your dash with a marker.

To look at it another way, lower tire pressure or combustion efficiency causing decreased fuel economy would definitely increase fuel injector pulse by requiring the driver to increase throttle or RPM (otherwise fuel economy would stay the same).

[benfrogg]

I will look into fuel pressure. I don't have a pressure gauge but may be able to rent one. They are kind of expensive to buy. Although, I feel like I've needed it multiple times in recent years....

I'm still going to have the gas station give me "exactly 1 gallon" in a gas can. I'll take it home and make sure it's right with my measuring cup.
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