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Who Do I Believe? The Pump Or The Computer.


SM 2003
Full Member
Aug 4, 2002
South Texas
I just had my second (third if you count the dealer tank) gas fill-up on my 03 AV and something is weird! :( I really like the new Diver Information center (DIC) ;D in the 03?s, I started to use the personal trip information to watch the fuel economy of my truck. I didn?t care about the first tank cause I was driving much slower than I normally do for the brake-in. But this last tank, my second fill-up I did care about.

So I filled up at a Chevron station, one I have been using for years now. I like Techron and that is why I use Chevron for most of my fill-ups. The pump said I used 26.446 gallons of gas but the DIC said I used 25.4 gallons of gas. I set the DIC at my first fill-up and I drove 388.9 miles between the two fill-ups. I normally fill-up to the max, after the first stoppage of the pump I wait a few seconds for the gas to settle down and I start the pump back up to top off the tank.

Now who do I believe! If I believe the Chevron station's pump my gas mileages is 14.7 mpg and if I believe the DIC it is 15.3 mpg. Both of which is great gas mileages for a 6000 lb - 4 wheel drive Vehicle. ;D :cool:

So is the DIC in the new 03 right? Or is the pump right? ??? ???
Unless you have to use Chevron, I'd try to replicate the experiment using a different gas station (try to make sure the pumps look like a different brand)...

I have had (though I don't have an '03 DIC) had the same type issue. My rememdy?

1. I fill up the AV with it leaning (laterally, Passenger side lower than drivers side)
2. I fill up at the same station, same pump every time. I find that I get a more consistent fill this way...

The pumps have to maintain a department of weights and measures (or some FTC thingy) while you DIC does not.

So.... I would trust the pump...

I would even suggest a massive excel spreadsheet to track your DIC info versus the pump and math info.. See if it is at least consistently off...

Here is one that I use

Gas Mileage
The fuel sensor in your vehicle will only be able to monitor the level of fuel in your tank. Thus, if you top up the vehicle completely, it will appear to the computer that you put in less fuel than the fuel pump.

I also understand the other aspect of this question, that the computer monitors fuel flow and tracks it as you drive. I would think that this is also subject to being very slightly inaccurate. Your vehicle is moving and jarring and being bumped, and this can introduce many inaccuracies. The fuel pump, on the other hand, is stable. In addition, they also have many years of development over the engine computer in today's vehicles.

Just my two cents. Check with another fuel station, but I would bet that you will experience the same thing there. Keep us informed whatever you do.
Watch how fast you get the disclaimer if you attempt to pin the manufacturer down to the accuracy of the DIC. This is the same for all onboard DIC systems they are designed to provide estimated information only. I am sure that if you look up the information in your owner?s manual it will more than likely state that the readings are estimates. On the other hand, have you ever seen a sign on a gas pump that states that the fuel dispensed from this pump is an estimate ???
C'mon - you're talking about a half-mile per gallon - does it really matter? ???

I'm not being harsh (and don't want to sound critical) but it's insignificant besides, when you think about it - you're getting the gas mileage you're getting :)

As for which one to believe, go with the gas pump and your calculations. As long as you do things the same each time your numbers should be as accurate as you can reasonably expect them to be. As someone said (above), the electronic DIC is only for an estimate and won't be as accurate as the gas station's pumps.

Don't forget to enjoy the drive!
Magic Mtn Dan said:
C'mon - you're talking about a half-mile per gallon - does it really matter? ? ???

Don't forget to enjoy the drive!
I am not complaining about the gas mileages, I think it is GREAT for this BIG Vehicle we all drive :). And you are also right, the gas station's pumps are more accurate the DIC, the pumps are look at by the department of weights and measures to be accurate.

I going to keep records and over time we may have a clearer picture on the accurately of the DIC. :cool:

I agree with keeping track over time. What you will probably find is that there are two errors. One is a random error introduced by filling the tank to slightly different levels at each fillup (even if you try to do your best to keep it consistant.) These random errors will average out over time.

A second source of error is introduced by the way DIC is keeping track of fuel usage. If I understand it correctly (I could be wrong) it keeps track of fuel usage by counting how many times it fires off the fuel injectors. Each time it fires the injector, it tallies up another drop of gas. The actual amount of gas it fires out of the injector may not be the amount it assumes is being fired out. This should be reasonably consistant over time.

So, without going into a full-blown statistical analysis, if you average out your fuel pump statistics over time, it should even out to be the real actual usage. Then compare it to what the DIC estimate says over time, and you will probably find that the DIC is just a bit off. Who is right? I'll go with the fuel pump for a variery of reasons.

It all boils down to the old adage: a man with one clock always knows what time it is, but the man with two clocks is never sure. (The reason being that the two clocks never seem to show the same time.)

Finally, there is also the situation with the fuel level sensor in the tank. These are notoriously inaccurate. However, I don't believe it comes into play with the DIC measuring instantaneous or average MPG or fuel usage. However, it does come into play when estimating range to empty. You will notice that while it is OK for guessing when you will need a fillup, I wouldn't stake a long walk against it. If you're coming up on a gas station, and the sign says the next gas is 100 miles away, and the range to empty says 105 miles, I'd stop and get gas now.

-- SS
I just did my 3rd fillup in my '03 on Sunday. I was actually surprised. Both times I've filled it, it's been within a gallon of what the DIC said I'd used. I was even thinking to myself that it was pretty accurate for just estimating.
Gas mileage?

When I am a long trip I put gas in the tank when the get fuel light comes on. I know that may not be the best way. At least I get the longest bang for the buck. I do not like to stop for gas on the road unit I have to. Anyway I have two 5 gallon gas cans in the back of the AV just in case.

p.s. no computer.
I have to agree with everyone else on this subject. The computer in the truck is an estimator that is close, but never exact. It just does not have the ability to finely gauge the use of fuel, or the amount in your tank.

Just keep your own log if you want / desire more accurate readings. If it is more of an experiment to find out where the limits of the truck is, just reduce the distance to empty by 10% and you should be OK every time. That is the rule I used on my last vehicle with the on board computer.

As far as filling up, then topping off the truck - be careful about this. Doing so can damage the fuel system if you put too much fuel in it. That is why the manuals state to stop when the pump first clicks off. Too much fuel can clog the emissions components attached to the tank lines and the tank itself causing the truck to run off of bad data form the sensors. Don't ask how I know this :)
How DO you know?? I am really curious, because I always beleived that they said that to "Avoid a dangerous condition" as posted on the gas pumps

What emmission components are we talking about here?? Can someone else confirm "Perpendicular's" thoughts on this??

I don't want to fill up to the "click off" because the last time I got my first click off, I put 3.5 more gallons in!!!