2025 GMC Sierra Owner Says His New Diesel Gets “Shockingly Worse MPG” Than His 2020 Model, Dropping from 24 MPG Highway to 17, and from 15 MPG City to “Just 9 MPG” After Only 9,000 Miles

There’s something quintessentially American about dropping $90,000 on a luxury-grade diesel pickup and then being surprised when it burns fuel like a 1970s muscle car. It’s the modern paradox of full-size trucks. We want torque, tech, room for the dog and the baby seat, off-road capability, and fuel economy numbers that wouldn’t offend a compact sedan. But every vehicle is a negotiation with physics, and the 2025 GMC Sierra 3500 AT4, glorious as it may be, is not exempt from the laws of mass and thermodynamics. One new owner recently learned this the hard way, and then did what every baffled driver does in 2025: he turned to Reddit.

Posting under the handle ResponsibleHope3880 in the r/gmcsierra subreddit, the owner detailed what can only be described as a disheartening experience with his new L5P Duramax-powered Sierra. 

“How is my brand new 2025 L5P getting worse mileage than my previous 2020? In my old truck, I was seeing averages anywhere from 18 to 24 miles per gallon on a ~50-mile freeway trip that I make frequently. On a 500+ mile road trip, I was hitting the 30s. I’d say I could easily get 15 in the city on any given week.

Now this new truck, I only have 9k miles on it, but that same ~50-mile freeway trip is barely hitting 17, and I pulled in from dropping my daughter off at school this morning, and I looked over to see 9 something. I love my truck, but this takes away from one of the main benefits of paying extra for a diesel truck.

Hopefully, this all makes sense. If you have any questions, please let me know. Thanks in advance.” 

The comment section delivered what only Reddit can: the full spectrum of responses ranging from sincere diagnostic tips to Zen-like sarcasm. “Did you buy this $90k truck for the gas mileage?” asked Realestateuniverse, cutting straight to the philosophical contradiction. It’s a fair question. The AT4 is not some stripped-down farm tool. It’s a luxury fortress with off-road credentials, massive curb weight, and the aerodynamic grace of a brick in a headwind. Nobody expects miracles at the pump, but compared to a previous model that hit the 30s on the highway, the new numbers sting.

GMC Sierra Diesel: Why Not Switch To An EV? 

• The diesel-powered 2024 model of the Sierra (with the 3.0 L Duramax turbo-diesel) delivers fuel economy of up to roughly 23 mpg in the city and 29 mpg on the highway for the 2-wheel-drive version. 
• A major recall affecting diesel-equipped trucks and SUVs from the parent company covers around 461,839 vehicles due to a faulty transmission valve that could cause rear-wheel lock-up and increase crash risk. 
• Some owners are switching to electric vehicles because even with decent diesel efficiency, the maintenance complexity, fuel cost volatility, and evolving regulatory headwinds make EVs increasingly attractive. 
• The diesel Sierra remains a compelling choice for towing and heavy use, but the shift toward EVs is driven by expectations of fewer moving parts, lower long-term operating costs, and future fuel/ emissions advantages.

Blame lies not in marketing or manufacturing, but in the reality of modern emissions systems, gearing, tire size, and sheer weight. Trucks have gotten heavier, not lighter. Software now calibrates for emissions compliance before fuel economy. Add to that rolling resistance from upsized all-terrains and a little extra eagerness from the driver’s right foot, and you start to see where those missing miles per gallon may have gone. Plane-Shallot-8326 summed it up succinctly: “After spending this kind of money, who gives a crap about mpg. Beautiful truck though.” Beneath the snark is a truth: no one is actually surprised by poor mileage in a 7,500-pound truck. They’re surprised when it’s worse than before.

Yet some helpful advice did trickle through. Troutman86 offered the most grounded response: “Proper PSI, less right foot.” In six words, he covered two of the biggest variables in fuel economy, tire pressure, and driving behavior. He added context from his own rig: a 2021 AT4 3500 running on 35s, which returned “around 14–16” on mostly highway trips. Not great, not terrible, and not far off from what the new owner is seeing. The original poster agreed: “Ultimately, it's more than likely gonna be the biggest factor lol.” In that moment, the conversation shifted from complaint to education.

But this isn’t just about one owner and his MPG drama. It’s about a broader truth in automotive culture. We are living in an era where every vehicle, from muscle car to mall crawler, is expected to perform every task brilliantly while sipping fuel like a hybrid. A Hellcat will chug gas at 13 MPG without apology. A Z06 Corvette will flatten both the road and your fuel budget. Nobody blinks. But a diesel truck that used to get 24 on the highway now returns 17? That gets attention because it challenges the long-held belief that diesels are the efficient choice for serious drivers.

And they still can be. The Duramax L5P remains a powerhouse of torque and longevity, designed for towing and long hauls. But every modern truck, especially the heavy-duty kind, is now burdened by more sensors, safety gear, larger wheels, and power-hungry accessories. Efficiency has become a casualty of capability and comfort. And while the EPA doesn't officially test three-quarter-ton and one-ton pickups, the owners do, every day, on the same roads they’ve always driven. When the numbers come back worse, it feels personal.

Despite the Reddit roasts, the takeaway is not ridicule. It’s realism. If you want a big, beautiful GMC Sierra diesel truck with heated seats, off-road chops, and the ability to haul your camper through the Rockies, then you have to accept the price at the pump. That doesn’t mean the original question was foolish. In fact, it was essential. It sparked a conversation that included humor, insight, and a reminder that buying power means owning the cost of that power. As Fhksws said simply, “God, that truck is beautiful.” It is. But beauty, as always, has its price.

Image Sources: GMC Media Center

Noah Washington is an automotive journalist based in Atlanta, Georgia. He enjoys covering the latest news in the automotive industry and conducting reviews on the latest cars. He has been in the automotive industry since 15 years old and has been featured in prominent automotive news sites. You can reach him on X and LinkedIn for tips and to follow his automotive coverage.