Bogus fuel mileage reporting. Grrrr.....

belchfire · January 28, 2021

Have any of you noticed that the reported fuel mileage is much higher than reality? I track every drop of fuel my truck burns and my hand calculations are always a fraction of the fuel mileage that the computer in the truck is reporting.

Just for the sake of disclosure, I run 275/70/18 tires and I have used the Rough Country speedometer calibrator and my speed is dead-on with my hand-held GPS, so that's not it.

Have any of you guys noticed that? Does it make you mad?

Edited January 28, 2021 by belchfire

m3n00b · January 28, 2021

Have any of you noticed that the reported fuel mileage is much higher than reality? I track every drop of fuel my truck burns and my hand calculations are always a fraction of the fuel mileage that the computer in the truck is reporting.

Just for the sake of disclosure, I run 275/70/18 tires and I have used the Rough Country speedometer calibrator and my speed is dead-on with my hand-held GPS, so that's not it.

Have any of you guys noticed that? Does it make you mad?

I've noticed this on all of my cars.

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Nanotech Environmental · January 28, 2021

39 minutes ago, belchfire said:

Have any of you noticed that the reported fuel mileage is much higher than reality? I track every drop of fuel my truck burns and my hand calculations are always a fraction of the fuel mileage that the computer in the truck is reporting.

Just for the sake of disclosure, I run 275/70/18 tires and I have used the Rough Country speedometer calibrator and my speed is dead-on with my hand-held GPS, so that's not it.

Have any of you guys noticed that? Does it make you mad?

I've checked mine a couple times and it was pretty close both times.

scrapen · January 28, 2021

Yep I noticed this first time I tracked a long road trip. I know when I see 20mpg in the summer it’s only 18-18.5.

Driving a vw diesel, I’ve gotten used to the “I get 55 mpg” when Fuelly website proves users typically get 40-45 on average with said engine.

belchfire · January 28, 2021

26 minutes ago, m3n00b said:

I've noticed this on all of my cars.

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I can't say for any other brand, but I first noticed it on my 2009 Silverado and that made my check my wife's 2010 Traverse. Now, it's confirmed on my 2018 Silverado and my son's 2020 Canyon. With a four-for-four rate in a four car family, it's clear to me that it's not a result of normal fluctuations, as they are all 8-10% high; occasionally higher, but never lower. GM could fix this, but I'd bet they don't want to.

m3n00b · January 28, 2021

I can't say for any other brand, but I first noticed it on my 2009 Silverado and that made my check my wife's 2010 Traverse. Now, it's confirmed on my 2018 Silverado and my son's 2020 Canyon. With a four-for-four rate in a four car family, it's clear to me that it's not a result of normal fluctuations, as they are all 8-10% high; occasionally higher, but never lower. GM could fix this, but I'd bet they don't want to.

My BMW, Mercedes, Mazda, Chevy and Honda all are about 1-2MPG optimistic.

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MaverickZ71 · January 28, 2021

1 hour ago, belchfire said:

Have any of you noticed that the reported fuel mileage is much higher than reality? I track every drop of fuel my truck burns and my hand calculations are always a fraction of the fuel mileage that the computer in the truck is reporting.

Just for the sake of disclosure, I run 275/70/18 tires and I have used the Rough Country speedometer calibrator and my speed is dead-on with my hand-held GPS, so that's not it.

Have any of you guys noticed that? Does it make you mad?

We also have an 09 and an 18 in the family. Both 5.3L crew cabs, bone stock. The 09 is a 6 speed Z71 4x4 and the 18 is an 8 speed 4x4. The window sticker on the 09 said 14 city and 19 hwy and it gets 10 city winter (remote start) and 12 city summer and 17 hwy. The highest I have ever seen was 18-19 hwy with a strong tailwind. Ford products I had driven in the past had been spot-on, so I attempted to have the 09 Silverado's DIC recalibrated under warranty when it was new. The dealer service manager there was no way to do so, and in his opinion, they were factory calibrated to be overoptimistic by GM to give IGMO owners who never hand calculate their fuel mileage that warm fuzzy feeling. I wondered if GM did used their 3.08 hwy gears/hwy tires/2WD calibration on all of the DICs, so if you're running 3.42 gears and all-terrain tires, the DIC reading is off. Also, I turned off AFM because of the excessive oil usage problem, which according to GM , is supposed to result in 7% worse fuel mileage. I didn't see any difference in fuel mileage after it was turned off. I hardly ever drive the 18, but now you've got me curious, so I want to take a closer look at what it is reporting and if it is accurate at all.

aseibel · January 28, 2021

In my 2015, the DIC reported average economy per tank is usually just a couple tenths higher than hand calculated. I have 3.42 gearing.

Not a big enough error to anger me. In fact its pretty close, so that I can actually trust it, just knowing its maybe 2% high.

I'm not sure if it keeps track of fuel used when remote starting, I suspect it only knows when you have the key in. So my guess is if you never remote start at all, your numbers would be much closer.

Edited January 28, 2021 by aseibel

belchfire · January 28, 2021

37 minutes ago, aseibel said:

In my 2015, the DIC reported average economy per tank is usually just a couple tenths higher than hand calculated. I have 3.42 gearing.

Not a big enough error to anger me. In fact its pretty close, so that I can actually trust it, just knowing its maybe 2% high.

I'm not sure if it keeps track of fuel used when remote starting, I suspect it only knows when you have the key in. So my guess is if you never remote start at all, your numbers would be much closer.

Mine is consistently 8-10% (and occasionally 12%) high. That's WAY out of line for a $40,000 automobile. It's really a very simple calculation.

As for remote start; the calculation is nothing more than miles driven divided by fuel used, so the remote start capability would be absorbed with no distinction. I assume that the fuel flow is mis-calibrated, because my speedometer is spot on (assuming the odometer is in tune with the speedometer). And I'll bet that this mis-calibration is intentionally high as MaverickZ71 noted "to give us all a warm and cozy feeling" and to fuel the bragging rights around the lunch table. I love this truck, but that one feature grinds my gears because I'm all but certain it's 1) correctable and 2) intentionally high.

LDM · January 28, 2021

Same issue on both my 2015 5.3 and 2018 6.2 Sierras. Mileage is off by 0.8 to 2.2, more accurate in the winter and less accurate in the summer. I had calculate every time I fill up just to see how far off it is. My 2018 is almost always 2.0-2.2 except in the winter, when its more like 1.3-1.5 off. I wish there was something the PCM/TCM/BCM programming that we could look at to see how it does the calculation and why it is so far off. Its almost as if the calculations is ignoring some portion of the fuel usage, like when idling while in drive or something.

aseibel · January 28, 2021

1 hour ago, belchfire said:

I assume that the fuel flow is mis-calibrated, because my speedometer is spot on (assuming the odometer is in tune with the speedometer). And I'll bet that this mis-calibration is intentionally high as MaverickZ71 noted "to give us all a warm and cozy feeling" and to fuel the bragging rights around the lunch table. I love this truck, but that one feature grinds my gears because I'm all but certain it's 1) correctable and 2) intentionally high.

Are you looking at the instantaneous mileage or for an entire trip/tank?

If the goal was simply to record the average fuel economy per tank of gas, yes it would be simply = total miles driven/fuel used.

But you know the DIC also displays real time fuel economy. So at any instant it is dividing speed by fuel usage to come up with a number. I have wondered if the "per tank" number is just a sum of all those instantaneous numbers. Notice how there is no display page to tell you how many gallons of fuel were burned? I don't believe the computer keeps track of that number. I wonder if anyone on this forum knows the exact math the DIC is doing.

When you hand calculate, are you simply taking the amount of fuel your receipt says you added to make the tank full? do you subtract the amount of fuel burned while idling, remote starting, sitting at stop lights, etc? If you don't exclude all the fuel burned while not moving, your hand calcs don't accurately reflect the in motion fuel economy either. Your hand calc numbers just represent an average for your style of driving.

You would have to burn up an entire tank on the highway with no stops to get a real accurate result, and I bet if you did that, the DIC number would be pretty dang close to your calculated number. I've done similar to that a couple times a year when we drive to the in-laws out of state. I drive mostly highway miles, only 4 stoplights in my town. I know the numbers are pretty close for me. I don't know your driving style to guess why your numbers are so far off.

At the end of the day, which number means more to you? The total miles you can drive on a tank (hand calc) or the instantaneous fuel economy while driving on the highway? I'm not saying either is wrong, but they aren't necessarily measuring the same thing.

Edited January 28, 2021 by aseibel

VanguardK2 · January 28, 2021

Have any of you noticed that the reported fuel mileage is much higher than reality? I track every drop of fuel my truck burns and my hand calculations are always a fraction of the fuel mileage that the computer in the truck is reporting.

Just for the sake of disclosure, I run 275/70/18 tires and I have used the Rough Country speedometer calibrator and my speed is dead-on with my hand-held GPS, so that's not it.

Have any of you guys noticed that? Does it make you mad?

This is kinda funny to see as how I've tracked every drop, and every penny for the last 6 years and 3 vehicles I've owned. My truck is also 1-2mpg higher and I run factory 265/65/R18 but K02s and a range afm disabler in Denver. Since the range disabler has been installed I've gotten about 30miles to the tank less than before. When I bought my truck in September 2019 it got 400 miles to a tank with 4 gallons to spare. With the all terrains, range AFM tool, and heater in the winter time like now, I will just barely break 340 per tank.

Unfortunately it all adds up but I'm savoring what good mileage I have now before I throw icon suspension, bed rack at 35s on it.

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CamGTP · January 28, 2021

Mines very close if I track it in the summer time, within 1mpg all the time. I get 7-10 mpg in the winter because I remote start it twice a day everyday and don't care at all.

Eddie 70 · January 29, 2021

Why would it make me mad?

I have never had a vehicle that provides a MPG readout to be correct. Get an app and keep up with it. You can see in my sig what my lifetime MPG is with my truck.

Booger T · January 29, 2021

I drive a 2020 3.0 and check my fuel mileage every fill up, I have found that the computer is usually reporting lower than hand calculating by about 1-1.5 mpg. I have only once had it hand calculate less than what computer was showing, sometimes it dead on but most times computer shows less than hand calculating.