Big mpg diff between DIC & manual calc on 04 duramax

[Babyhauler]

This is on my new to me 04 2500HD 4x4, crew cab, Duramax, 195,000 miles. Edge tuner set on mileage, dual exhaust w cat delete, Bully dog cai, 265-75-16 BFG AT tires. Runs & drives fine.

 

My first tank of fuel was showing 16.9 mpg at fill up. Hand calc indicated 12.4 mpg. Big difference. My many other chevy trucks & Tahoes would usually be about .6-1.0 mpg higher than manual calc. I have never had one be this far off. Is the tuner causing this or some other setting? The fuel guage fell really fast after 1/3 left. I don't know if that has anything to do with it or not. I don't think so.

 

Let me know if there is a solution for this big difference.

 

Thanks for the help.

[gmtech4]

I could care less about mileage , it's the cost of driving ,

 

Don't loose your mind doing BS manual calculations , it'll make ya nuts imo

[Babyhauler]

I am not really concerned about fuel mileage on this post, just the calculation difference.

 

I don't think there is ethanol in diesel fuel?

 

I only buy non-ethanol gasoline for my power equipment.

 

I usually buy fuel in the morning to get the most pounds per tank since it is denser when cool.

 

I usually see a .6 - 1.0 mpg difference in hand calculation vs dic on my other vehicles. Winter or summer.

 

This tank full was a 4.5 mpg difference. About 25 percent lower than dic. I'm wondering if something is not operating properly to cause this reading. No leaks or fuel smell in oil.

 

Any ideas concerning this difference?

 

Thanks.

[Babyhauler]

I have no problem with the cost of driving, I just want to get the most bang for my buck. As a small business owner, fuel cost can be a large expense so I need to monitor it like any other expense. I'm keeping up with my miles for taxes, so I may as well calculate it. I have done it for 35 years. It's kind of like weighing yourself every day, you can make a change before your clothes start getting too small. It can help indicate if something is off a bit, such as O2 sensors for gassers, real world diff for tire changes, loaded or empty. It helps with diagnoses of a problem instead of "it seems to be using more gas". If I was drag racing at the track, I would record my time slips and notate what changes helped or hurt. Same idea, just not as fast or exciting.

 

If my cost per mile with the diesel is the same as the 6.0, with better towing performance, I guess I have improved my situation. I was hoping for a little higher mpg in the empty/light towing mode. The lower than expected mpg info is for another post or search.

 

The question for this post is why is there such a difference between the computer mpg and hand calculation on my first diesel truck? Is something broke or need adjustment? What sensor does it calculate mpg from? Can I fix it or make it better?

 

Thanks for the replies.

[Doug_Scott]

I would expect if something was causing that much of a difference it would set a code. To be that far off, it would have to be out of spec. Did not notice the Duramax part. But, diesel fuel is still changed for different seasons. This time of year is when the change is most likely to happen.

 

Are you other vehicles diesel as well? If not, you can't really compare them to each other. Depending on how deep their tank is in the ground, time of day may not matter much regarding temp of fuel in tank. Around here the tanks are a minimum of 10 feet down, our frost line is about 1.2m. Below that is not far off the 15 degrees C.

[00Silverado4x4]

Are the tires the stock size? If they aren't, that would be the reason I think.

[Jsdirt]

The reason they're so far off is, the average mileage is calculated from the time it was last reset - that could be a week ... or 10 years ago. The longer the time, the wider the average is.

 

So, if you got 10mpg one day towing, and 19 another day on the highway, those peaks and valleys will be smoothed out over a longer timeframe. Calculating by hand tells you your MPG over the course of a tank of fuel, if you did it after a fillup - much shorter time frame. More realistic too.

[Doug_Scott]

There are far too many variables that effect fuel mileage. Some you have no idea are even there. You can get one tank of fuel one week that has winter additives in it, then the next tank does not have them. You will see a fuel mileage change. Using your example of watching your weight by weighing yourself daily should actually show you that doing that will drive you nuts trying to keep your weight under control. My weight can change by 5 pounds in a 24 hour period.

 

The mileage is not calculated by just one sensor. Have you ever checked the fuel used number in DIC? I filled my truck up yesterday, and put 81 litres in it. DIC said I had used 76 litres. That's a just over one gallon difference(canadian gallon is 4.54L). On a 20 gallon tank, that is 5%, so if I was hand calculating that one tank it would show 17.8L/100km compared to the displayed 17L/100km. One more thing you need to remember when doing tank by tank calculations, is that the fuel you pump is being measured as though the temperature is 15 degrees C (in Canada). The pump may say you got 19 gallons, but, depending on actual temperature of the fuel in the ground and temperature of the pump may make that volume change. Remember that liquids shrink when cold, and expand when warmer. You can also apply that to what the PCM says you used. What temp does the PCM use?

 

I think you need to find an acceptable range to accept. The difference may be a simple as the PCM takes into account the temperature of the fuel being used, and you cannot possibly do that in manual calculations.

[Doug_Scott]

MPG numbers are relative. There are a couple of ways to look at it. As Jsdirt said, manual calculation of last tank is more realistic of what is happening today.

 

Resetting the MPG and watching it from that clean reset will show you instantaneous MPG but only for about 5 minutes, then the average starts taking over. The change in MPG becomes less dramatic. By the time the tank is empty, the MPG will be pretty steady, but will still show a change if you floor it for a mile or so. If you do not reset the DIC for 6 months, and recorded mileage and fuel used then i would expect the DIC reading to be close to the manual calculation.

 

I know a guy that has never reset his DIC and has kept complete records of fuel purchased, and distance traveled due to his work paying a per mile expense. He figured his mileage for the last 42,000km, and the DIC is reading 0.6l/100km higher than manual calculations. To me that is close enough. His average mileage is 15.4l/100km with about 30% hwy, and driving in 4 distinct seasons(temp range from 35 degrees C to -30 degrees C). 2012 Sierra 5.3 Ext Cab, 4x4, one trip towing, approx 5k lbs and less than 500km.

[Jsdirt]

If you live in MA, you can have anywhere between 10 -25% ethanol in your fuel ... PLUS it will vary from week to week at the same station. It's hit or miss.

 

I can always tell when I have a crappy batch of fuel in the first 3 miles, or the first time I stomp the pedal to pass someone after a fillup. High ethanol concentrations rob horsepower & kill fuel mileage, even though this one was designed for E85. I normally average 16 MPG, but I've had tanks of gas where 14 was the best I could do. 3.73's out back. One time my scan tool read 19.7% alcohol content on one particularly crappy tank. Everyone wins on that EXCEPT us - your fueling up more often, making the gas station and refiners more money, and don't forget your also paying taxes on that fuel, so the state and feds win too.

 

I'm gonna stop there, or I'll wind myself up into a rant ...

[Jsdirt]

Mine has been fluctuating between 14.3 & 14.6 forever. I think the last time I reset it was back in winter '07.

[Doug_Scott]

Thanks for the replies.

I guess I should have added that I reset the mileage when I filled up right after I bought it. Just like my other vehicles, I reset it every tank and record my mileage manually. Years of mileage & fuel records for tax purposes. I've just never had one have that big of a difference. Usually about .6 - 1.0 difference on others. Speedo is spot on with my gps. 265 vs 245 tires only made a slight difference.

I wonder if the tuner has any effect on the reading? Where does it get the amount of fuel used to determine mpg in the computer and could that part be defective?

The search continues...

 

I would imagine that they use the statistics from the injectors, engine rpm, engine temp, air temp, etc to calculate mileage. One thing that can effect the mileage is the amount of ethanol in the fuel. I don't know if it is capable of detecting small changes in the amount of ethanol in fuel(10% or 15%) like it can for detecting E85. It should be pretty easy to measure fuel used accurately, given the data available to the PCM.

[Doug_Scott]

Pumps here say fuel may contain a maximum of 10% ethanol. I am not convinced the PCM(or is it ECM?) are programmed for variable ethanol content. They say that E85 is supposed to provide more power, but I, like you, have found that the mixture stuff kills power and mileage. Back when I had my 2000 GTP we had 94 octane fuel here at Sunoco. We also had 91 octane at Esso. The 94 octane would give me 10 to 15% lower mileage than the 91 octane. Back then the 91 did not have ethanol, 94 did. They got the 94 rating by adding the ethanol.

 

Something I have noticed in the last few years that just because it says Esso on the sign, does not mean the fuel comes from Esso. I have seen Esso fuel transports on the highway, and have seen off brand transports dropping fuel at my local 24 hour Esso station, usually after 1am. But then again, I also cannot figure out how a fuel transport can drive right by the exit at the highway here, continue on another 50 miles and drop fuel at a gas station that is selling that fuel for 10 cents a litre less than it is in the Toronto area. 10 cents a liter is about 45 cents a gallon here. Those stations are just far enough away to not make it worthwhile to drive there for fuel.

[Babyhauler]

Thanks for the replies.

I guess I should have added that I reset the mileage when I filled up right after I bought it. Just like my other vehicles, I reset it every tank and record my mileage manually. Years of mileage & fuel records for tax purposes. I've just never had one have that big of a difference. Usually about .6 - 1.0 difference on others. Speedo is spot on with my gps. 265 vs 245 tires only made a slight difference.

I wonder if the tuner has any effect on the reading? Where does it get the amount of fuel used to determine mpg in the computer and could that part be defective?

The search continues...