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In a surprising move, the 2021 Chevy Silverado 1500 Duramax diesel will see a big drop in the stated MPG ratings versus the prior year, and nothing significantly changed from the 2020 model.

So, what’s up?

GMAuthority.com discovered this news after carefully examining the stated MPG numbers for the 2021 models. They noticed a pretty big difference for the 4WD version of the pickup versus the prior year.

Specifically, they found:

2021 chevy silverado 1500 duramax

GM Authority reached out to Chevy and got the following reason for the drop:

“The 4WD 3.0-liter configuration went up in its test weight class, which led to slightly reduced fuel economy to 22 city/26 highway/24 combined. The 2WD configuration remains best-in-class with 23 city/33 highway.”

Huh? Confused? Let us explain.

How EPA testing works for trucks like 2021 Chevy Silverado 1500 Duramax diesel

First, GM Authority followed up and got this additional tidbit: It was a “higher-contented truck being tested based on production schedule.”

Unfortunately, GM Authority didn’t really understand what this meant and closed with people won’t see much difference in real-world testing.

This is the wrong conclusion, here’s why.

The MPG estimates come from Environmental Protection Agency (EPA) testing, which GM conducts on its own vehicles. Basically, the automakers self certify their own results and report them to the EPA. The EPA can — and has — done spot testing to make sure the automaker is, well, telling the truth.

Part of this testing allows automakers to pick the vehicles they want to rate for the EPA based on what they think will sell. For example, if GM thinks its best-selling Silverado is going to be the LTZ model, it will test this model and send the results in.

This is where it gets tricky.

Worse MPG! 2021 Chevy Silverado Duramax diesel 4wd sees big drop

This Chevy Silverado Trail Boss won’t get the same fuel economy as a base model. Why? Lift and tires. (Photo courtesy Chevy)

Most consumers know when you lift a truck and put bigger tires on it, you get worse fuel economy. It is physics. Automakers know this, too, and they often rate their off-road variants with the same fuel economy of the model they want the EPA ratings for. This makes them seem more competitive on paper with their rivals while explaining to consumers the EPA numbers are “estimates” and they shouldn’t be taken for fact.

There have been lawsuits over this as you would imagine — like the Ford Ranger one we covered a few months ago.

With more stock, off-road configured trucks coming to market, many automakers are getting a bit smarter, trying to avoid lawsuits, and are rating this trucks with a higher rating. This also applies to trucks with more bells and whistles.

So, what happened then? GM guessed consumers were going to buy the lower trim level of the Silverado with the 3.0-liter Duramax diesel engine, and instead, consumers bought a higher trim level. In order to stay EPA complaint, GM revised its numbers to reflect what consumers were buying, and the additional weight of the more options on this truck bring down the fuel economy.

Make sense?

The bottom line 

While, yes, EPA estimates are just that, estimates, they were created to give consumers guidance on what they could reasonably expect to see for fuel usage when they bought a new truck. The recent controversy over VW and Ram with its EcoDiesel has made the EPA more stringent — aka pissed off  — and automakers are paying more attention to what numbers they put on the sticker.

This is good for them and good for you, the consumer.

Related posts:

Ford F-150, Ranger mired in new fuel economy lawsuit

U.S. Justice Department Demands FCA Recall EcoDiesel, Pay Fines

Shocker! 2019 Chevy Silverado 1500 MPG vs 2018 Chevy Silverado 1500



Tim Esterdahl

Automotive Journalist Tim Esterdahl has been a lover of trucks and SUVs for years. He has covered the industry since 2011 and has pieces in many national magazines and newspapers. In his spare time, he is often found tinkering on his '62 C10 pickup, playing golf, going hunting and hanging out with his wife and kids in Nebraska.

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  1. Brandon October 14, 2020

    Interesting. I’m wondering if the drop in fuel economy rating is related to the substantial price drop. It would kind of make sense to lower the price if the fuel economy rating is really not as good since that is usually the reason to buy a half ton diesel.

  2. ROB WILHOIT January 6, 2021

    I just bought a 2wd RST crew cab in TX and drove it home to S FL. I averaged 27 mpg.

  3. Aaron Rimpel April 16, 2021

    I just bought a 2021 Silverado LT Crew Cab 4wd Z71 as a daily driver. I do about 50 mi/day round-trip to work, so I got mine with the 3.0 Duramax and 10-speed. I’ve actually been shocked how well my fuel economy has been in the short time I’ve owned it. I just filled it up for the first time with 600 miles on the odometer — i.e., drove it off the lot with low single-digit miles and a full tank and just filling it up again. The trip computer says I’ve averaged 26.2 mpg with mixed driving. My best round trip from work (missing most lights) was over 29 mpg. My “best” mileage for 25 mi was 33 mpg according to the computer — I’m confident that I can average over 30 mpg highway based on my current data. I’m extremely pleased with this, especially since the EPA numbers are 22 city/26 highway/24 combined for the 4wd.

    1. Jerry Kirwan January 29, 2022

      I have a 2020 GMC AT4 with the 3.0 Duramax and I can tell you I have never seen MPG approaching 30…and I do a lot of highway driving. I have had it for 35k miles and i average around 21 MPG overall

    2. wasj01 April 4, 2022

      You will probably not get this kind of mileage consistently. The mileage will drop when the engine goes into regen. I average 26.5 on the highway. It gained about 1 mile after the 15,000 mile break-in period. This was from 25.5 to 26.5..

  4. Ryan February 11, 2022

    does anyone have 3.0 lifted with 35″ tires or bigger. I am considering this option but would like to know the MPG with bigger tires.

  5. Ray J November 8, 2022

    I have a 2021 GMC Sierra with the six cylinder turbo diesel. I was getting 36mpg highway, but after the dealership did a GM recommended reprogram, due to the DEF gauge not reading properly, I now get 26mpg. 10mpg drop off, and the dealership has no idea why.??? That is costly…..


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