Accuracy of Gas station pump
 

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

Re: Accuracy of Gas station pump
 

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

Re: Accuracy of Gas station pump
 

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.
B

Re: Accuracy of Gas station pump
 

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?

Re: Accuracy of Gas station pump
 

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

Re: Accuracy of Gas station pump
 

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

Re: Accuracy of Gas station pump
 

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

Re: Accuracy of Gas station pump
 

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?

Re: Accuracy of Gas station pump
 

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

Re: Accuracy of Gas station pump
 

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.
B

Re: Accuracy of Gas station pump
 

I got this one for $20, although it's $25 now:
https://www.harborfreight.com/fuel-in...ter-92699.html
It worked for my 2002 GMC, but I don't know if it fits whatever's in the VX.
"Fits most systems (except for Bosch, CIS-Jetronic and GM throttle body systems)"

Re: Accuracy of Gas station pump
 

I too think you might have a fuel pressure difference via fuel regulator or wiring or fuel pump voltage change?

On all my Hondas and DSM's its one of the first mods I do is re-wire the fuel pump with a bigger gauge wire.

Re: Accuracy of Gas station pump
 

So, I'm still all about the fuel pressure side of things, but here's some news.

I pumped one gallon into a gas can. I brought it home and measured it with a 2 cup measuring cup. There are 128 ounces in a gallon. That's 16 cups.

Perplexingly, I got 17 cups of fuel.
Now, I pumped very slowly (not at all on the fill up setting). I also got less than the normal fill up. (only one gallon)
This tells me I'm on to something....
Pumping slowly may change things. Look at this article:
https://www.npr.org/templates/story/s...toryId=5304258

I found this:
https://www.bimmerfest.com/forums/sho...d.php?t=139828

And a few other articles skimming the net on the topic.
https://www.fredericksburg.com/News/F...5312001/296679

The stations typically dispense 1, 5, and 10 gallon increments correctly and other increments incorrectly. 1,5, and 10 are the normal testing pump numbers. So, those amounts are usually right on (within 5% accuracy).

I've been using the same pump for the duration of my gaslog, with only a couple of exceptions. A couple of exceptions that have been higher MPG too.

It seems there some weight to my argument about incorrect pumps.
B

Re: Accuracy of Gas station pump
 

Okay, so more reading has proven more results.
It seems that in Maine, testing is done with 5 gallon containers. All pumps are tested this way. That means, I'm very likely to get 5 gallons if I pump only 5 gallons. So, if I watch the MPGUINO till it says I've used 5 gallons (half a tankish) then go to another pump (or the pump in question) and my MPG's go up when I top off the tank, I know there's a scam going on.
My first boss is the guy who goes around and checks the pumps for accuracy. I need to look him up!
B

Re: Accuracy of Gas station pump
 

i do think pumping slowly makes a difference...https://www.gassavers.org/showthread....pumping+slowly

Re: Accuracy of Gas station pump
 

Wow that is amazing that you an entire cup extra by pumping slowly . . . Guess what I am doing the next time I fill up . . . Going for ten gallons and pumping slowly!

Re: Accuracy of Gas station pump
 

The weights and measures test is done at both the slowest stetting and the fastest. They must pass both. It seems filling slower does have some weight to it.
B

Re: Accuracy of Gas station pump
 

News-
Filled up last time at the new station. A sad 53mpg. But, I got a tank distance record on this fill up! That means that the old pump was not topping off nearly as much as the new gas station/pump. The new number was 64 mpg much closer to the MPGUINO. Let's see if it stays there! I'm psyched to see 64mpg on a 716 mile tank.
If a couple of tanks go by that are obviously better than the last station, a call to the sate is in order.
Turns out it pays to pay attention!
B