2021 Tahoe LT Duramax mpg/mpd increase after maintenance and software upgrade

[Alexnorton]

Last month I had my Tahoe 2021 LT Duramax for routine maintenance oil change in the dealer.
It's mpg/mpd changed from avg 30 to 26.
The dealer said they didn't do anything.
I'm not sure what I should do.
Does anybody have a similar issue?
Is it maybe because of bad injectors?

 

[DuraYuk]

Last month I had my Tahoe 2021 LT Duramax for routine maintenance oil change in the dealer.
It's mpg/mpd changed from avg 30 to 26.
The dealer said they didn't do anything.
I'm not sure what I should do.
Does anybody have a similar issue?
Is it maybe because of bad injectors?

No. There are 1000s of things that could have led to your decreased mpg. If you don't have any warning lights doubt it actually has a mechanical problem.

Just drive it and see. Check your tire pressure.

 

[B-train]

Also, has your fuel filling changed locations/vendors?

If they did a new engine calibration, then YES it could affect MPG. If they did that, it's most likely an emissions "upgrade" that as we all know is a complete joke. Think VW.....all the cars with "corrected" calibrations got worse MPG.

In the future, if it's running well, tell them no more refreshes without approval and letting you know what it's supposed to correct.

 

[StephenPT]

Going from straight diesel to B20 will give you that kind of drop in fuel economy. Do you keep track of where you fill up and did you fill up shortly after visiting the dealer?

 

[Alexnorton]

Thanks guys
Fortunately, I don’t have any warning lights on.
Tire pressure is ok
Didn’t change fuel type.
I drive it about 1000 miles which I did it more than 10 times in the past and filled the tank from the same gas stations. But the results are not favorable.
With a full tank I could drive around 767 miles and now it’s around 550 miles.
I’m not sure if they did any engine calibration, but definitely I feel it has more power and I can’t hear it’s turbo noise.
If anyone has any recommendation regarding the engine calibration or anything that can help I appreciate it to share.

 

[Stbentoak]

Well 550 to 600 is much more realistic. You can get 30, but to AVERAGE 30 is a high anomaly.
I avg 24-26 and I'm very happy with that...

 

[Brand0n]

767 miles?! Agreed 550-600 has been my experience…

 

[Alexnorton]

 

[DuraYuk]

Thanks guys
Fortunately, I don’t have any warning lights on.
Tire pressure is ok
Didn’t change fuel type.
I drive it about 1000 miles which I did it more than 10 times in the past and filled the tank from the same gas stations. But the results are not favorable.
With a full tank I could drive around 767 miles and now it’s around 550 miles.
I’m not sure if they did any engine calibration, but definitely I feel it has more power and I can’t hear it’s turbo noise.
If anyone has any recommendation regarding the engine calibration or anything that can help I appreciate it to share.

Your 24 gallon gas tank is averaging 777 miles? That's over 30mpg! That's crazy! I get 30mpg average on highway around 80mph.

 

[martinajm]

I barely get 700 miles on my 28 gallon tank in my XL. Seems to me like they fixed an overly optimistic fuel economy calculation. Do you have hand calcs for fuel economy before and after the change?

 

[jm98]

Hand calculate. That dash number changes at any given throttle imput.

 

[Tinker]

I haven't been on any recent trips. My city avg gives me about 600 miles per tank. On highway it's over 700. It has the latest updates and recalls and haven't noticed any drops... I'll pay closer attention though.

 

[petethepug]

That occurred after a dealer visit with my supercharged VW. After losing the performance in a few more months I discovered the leaky battery issue they claimed unable to duplicate was battery acid leaking down into a abs plastic boost hose.

The dealership had followed the acid leak to the boost tube and stuffed a rag into the hole eaten by acid. There was even a TSB out on that very condition.

Where I’m going with this is the possibility they fixed something off the books. Something they didn’t want you to know about or we’re trying to conceal.

 

[Stbentoak]

I could be wrong, but no one has ascertained this person has a 24 or 28 gal tank, right?

 

[Jellyfrosh]

The dash lies all the time. My last fillup it said I had a range of like 710 miles or something.

Could be a different diesel blend, either bio diesel or diesel number 1.
Possible they put the wrong weight oil in your engine, thicker oil would reduce economy.
Could be your tire pressure.
Could be that you had more regens this tank for any number of reasons.
Could be headwind.
Could be outside temps.

There's like a million other things it could be too. Check the easy stuff first.

 

[StephenPT]

I could be wrong, but no one has ascertained this person has a 24 or 28 gal tank, right?

OP said Tahoe, so it's a 24gal tank. 28gal is on the Burb/XL.

 

[Stbentoak]

OP said Tahoe, so it's a 24gal tank. 28gal is on the Burb/XL.

Well, I kinda see that, but that makes this less believable.

 

[R32driver]

The dash lies all the time. My last fillup it said I had a range of like 710 miles or something.

This is true. I pretty routinely see that screen say 495 to 505 miles available after a fuel up on my xl with a 5.3 and it does not get that many miles no matter what you do. Those numbers drop almost immediately once you start driving. I've never actually calculated it but we probably get a little over 400 miles on a full tank based on my lifetime avg of 17.5 mpg

 

[Alexnorton]

Thanks Guys for all answers.
Yes, I have Tahoe 24gl diesel.
I’m using it for the long trips not at all in the city.
And after some research I found, GM released another update in Feb 2023 that changed the previous vehicle parameters.
After another trip I found my mpg get much better than before.
I believe after every update, the vehicle will start to learn the driving parameter and adjust the mpg based on how you drive your vehicle.

 

[goldfinger]

The indicated range and mpg increased significantly after I had the update done a couple weeks ago, but I sold the car for a new 2023 a few days later so didn't get much of a chance to observe.