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How Accurate is your DIC MPG?


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So i finally foudn myseld a 2009 Avalanche and picked it up last weekend and just ran through my first tank of gas yesterday. When i test drove the truck it only had what the guage read as being about an 1/8th tank. the dealership did the first fill up so it was full when i picked it up.

 

Anyways. When i drove home my DIC read up to 20MPG down the highway. I probably could have gotten it higher if i set the cruise control but I started to hit heavier traffic. I generally do a mix between city/highway driving but the highway driving I do can sometimes have alot of traffic. Given the truck is rated at 15/21 i honestly assumed I would be in the 15-16mpg range but figured it might see 17 but i wasn't sure.

 

I was always skeptical of the dic mpg rating in the trucks. I know in the Impala I had before it, it always said 21.5 MPG or aroudn there but every time i filled up the car took around 13 gallons and i always averaged around 235 miles per tank which came out to 18-19mpg on the average. A buddy of mine has an 08 4sp w/ the 5.3 and he claimed his truck got 15mpg but also told me he was just going off his DIC. When i asked him if he had ever did the math he shook his head no but not long after said he did do the math and claimed it to be accurate. I was also skeptical of this since he added 285's and didn't do any re-calibration. I know the E-rated 285's really did a number on his gas mileage though.

 

Anyways that brings me to yesterday when i go to fill up. My guage read about an 1/8th tank and said i had 84 miles to go and i had 339.5 miles on the tank. I wasn't sure how much gas the tank was going to take before the pump click off. It took 25.5 gallons.

Now if you take (339.5/25.5) by the math I only got 13.3 mpg out of that tank and if you add the 84 miles to the 339.5 i drove and divide it by the full tank (423.5/31/5) I come u p with 13.4 mpg which i think is accurate. My DIC read 15.7 mpg.

 

 

Now, i did end up driving in more traffic than normal last week which accounts for the lower than average mpg but I'm over 2mpg lower on my calculations than what he DIC says, again doesn't surprise me too much. I really like the truck. It runs good, its comfortable, overall I'm very happy with the truck I was just a little surprised it wasn't higher. Either way it still gets alot better gas mileage than my 2000 Silverado 2500 w/ 285's.

 

Now one thing i must say is with the 6sp and 20" wheels the thing tends to coast on the highway alot more than im used to i find myself having to hit the brakes to slow down even when I'm coasting. Once i get used to the truck I could probably gain some fuel economy on the highway/city just by not having to hit the gas as much.

 

 

I was just curious if anyone else found the DIC to be inaccurate vs the actual math.

 

One thing i did do is I started a spreadsheet where I am going to keep track of the (Date, Mileage per tank, Gallons to fill, DIC MPG, Clacualted MPG, Price per Gallon, Total Cost, Gas Station, and a Comments section in case I want to add a note on that tank of gas) and actually see if I see any differences. In my past vehicles i always put around the same amount of mileage on the tank vs what it took to fill.

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Let us know what you find out. I have never hand calculated my MPG. I really don't care too much as even if I know the DIC is off, that alone isn't going to improve MPG. So it's not important to me. My truck just turned 9k miles and my DIC says 15.8 MPG overall. I tow a little and step on it once in awhile but not much.

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It will be probably 10 days hopefully till my next fill and then we'll see how it does. This tank might be a little more accurate because it will my normal routine driving as well as the fact I filled the tank. I was just curious what i was getting. Between driving back and forth to work today I'm back up to 15.8 again.

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Five years of data indicate the following on mine:

 

Gallons Used: DIC is 5.28% Optimistic

MPG: DIC is 4.46% Optimistic

 

Statistically, the spread is MUCH tighter on MPG than on Fuel used.

 

There is clearly a correlation between the Gals Used error and tank level when you fill. For example, If I fill up when the tank is exactly half empty, according to the gauge, the gals filled and gals used inidicated are very close (fairly consistently), and it gets worse when you fill at a lower or higher tank level, with the average error being what is indicated above.

 

The MPG statistical spread shows there is a truly near-linear bias (with a fairly decent correlation coefficient) of about 4.46%.

 

So, if the DIC says 15mpg, it is REALLY about 15/1.0446 mpg, i.e. about 14.35 mpg.

 

If it indicated 10 gals used, you probably used around 10*1.0528=10.52gals, on average.

 

Again, the data set is about 5 years of data at an average of 1 fill up per week, so about 260 fill-ups.

 

AND, yes, before you point it out, I have NO LIFE :crazy:

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Five years of data indicate the following on mine:

 

Gallons Used: DIC is 5.28% Optimistic

MPG: DIC is 4.46% Optimistic

 

Statistically, the spread is MUCH tighter on MPG than on Fuel used.

 

There is clearly a correlation between the Gals Used error and tank level when you fill. For example, If I fill up when the tank is exactly half empty, according to the gauge, the gals filled and gals used inidicated are very close (fairly consistently), and it gets worse when you fill at a lower or higher tank level, with the average error being what is indicated above.

 

The MPG statistical spread shows there is a truly near-linear bias (with a fairly decent correlation coefficient) of about 4.46%.

 

So, if the DIC says 15mpg, it is REALLY about 15/1.0446 mpg, i.e. about 14.35 mpg.

 

If it indicated 10 gals used, you probably used around 10*1.0528=10.52gals, on average.

 

Again, the data set is about 5 years of data at an average of 1 fill up per week, so about 260 fill-ups.

 

AND, yes, before you point it out, I have NO LIFE :crazy:

 

I keep tabs on a per tank basis but I've never gone as far as plot all this much information. I like it though, good work.

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Are you guys comparing the MPG by calculating how much gas you're putting in each time vs. what the DIC is telling you? How would you know if you're refilling with the EXACT same amout each time? Could that cause some of the variations from manually calculating vs. what the computer is telling you?

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Are you guys comparing the MPG by calculating how much gas you're putting in each time vs. what the DIC is telling you? How would you know if you're refilling with the EXACT same amout each time? Could that cause some of the variations from manually calculating vs. what the computer is telling you?

 

 

Good point/question. Plus is everybody using the same fuel pump at the same gas station everytime they fill up? Surely there may be some slight differences among pumps. I doubt they are dead accurate and doubt they all dispense the exact same amount each time. In either case, I know neither the DIC nor hand calculating are going to be perfect, so I again I don't worry about either too much. But looks like they are close from the post, so it's good info.

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Are you guys comparing the MPG by calculating how much gas you're putting in each time vs. what the DIC is telling you? How would you know if you're refilling with the EXACT same amout each time? Could that cause some of the variations from manually calculating vs. what the computer is telling you?

 

 

Your right but that is the idea of keeping track of everything in a spread sheet. Given the amount of miles I drive every week its going to be a few weeks at minimum till i really start to have any data to compare.

 

I generally fill up at the same gas station all the time. Its rare that I fill up elsewhere. I actually usually tend to use the same pump if its open. Overtime i came to know off the top of my head how many gallons of gas my other vehicles would take just based on what the gauge said. If your within a half gallon or even a gallon every time its not going to change the number mpg THAT much. That's also why I said this tank would be a more accurate representation of what I might expect because I filled the tank myself so I know exactly how much i put in.

 

Its not an exact science but In my case 13.3 vs 15.7 isn't even close which was the basis for the original question.

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I am sure no one is claiming that these are laboratory class measurments (mine certainly aren't). The error you refer to via "are you sure you're filling to the same level every time, etc" is a valid issue.

 

We tend to be creatures of habit. As long as the same person is doing the filling every time, the practice tends to be consistent, fill to fill. Thus, with a large enough data set, that error will tend to minimize itself and get lost in the noise. You are correct, however, that is an issue.

 

If the DIC fuel used error is on the order of 5%, that means that for the "fill error" to be a significant part of that 5%, you would be filling up every time +/- 1 gal (based on a 20 gal fill). I do not believe that my (consistent) fill-ups vary by as much as +/- 1 gal. If you fill to the first click, you have to try pretty hard to get an additonal gallon in there (at least that's my experience), and I don't really bother.

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My truck seems to be fairly close on the part that says gallons used. I did notice that I went 325 miles and the bell rang and the fuel light came on. The gas guage pointer was almost even with the E line. I drove another 8 miles and had 333 miles on the truck and it only took 21 gallons. I have a 26 gallon tank, which means I would still have 5 gallons left. The DIC said I only had 40 remaining miles. I hope I wouldn't use 5 gallons in 40 miles. Seems kind of odd to me that these numbers don't line up.

 

I have been getting anywhere from 16 to 17.8 mpg on my truck. I leave the total mpg alone and it says 16.9 right now. My truck has the 5.3L with the 4 speed with 3.73 rear end, so I may not get the best mileage some others do even though I am easy on the pedal.

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I have calculated by hand but I usually just go by the DIC with the understanding that it will be off a little depending on how I fill up. I started doing some testing of my own, and I read the owner's manual. The reading was a little hard as I graduated from a high school in Kentucky. Anyway, the manual states that you should stop filling up when the pump clicks off. I understood this to mean that I shouldn't try to squeeze in any additional gallon. According to the manual I have been putting too much gas in all my automobiles as I am always putting as much gas in as possible. The DIC was off by about 1-2 mpg when I put as much in as possible, but when I switched to stopping the fill when the pump clicked off the DIC was much more accurate, within 0.5 mpg. The actual gallons used was also much closer.

 

I live in plain old flat Indiana. If I drive on a country rd at around 50-55 mpg I will get between 21-23 mpg. If I'm on the interstate going 70 mph I get about 18-19 mpg, and if I'm goine 80 mph I get about 15-16 mpg. On the rare occasions that I'm in a great big hurry and I'm going 90-95 mph (I don't recommend) I get about 12 mpg. All this is approximate as the wind in Indiana is unpredictable and variable. Driving into a 30 mph head wind will have a huge effect on these numbers.

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Are you guys comparing the MPG by calculating how much gas you're putting in each time vs. what the DIC is telling you? How would you know if you're refilling with the EXACT same amout each time? Could that cause some of the variations from manually calculating vs. what the computer is telling you?

 

 

It doesn't really matter because of your driving is not always the same. The weather is different every day.

There will be always circumstances what changes the numbers.

 

so long

j-ten-ner

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