Chevy 6.6 Gas MPG: Real-World Fuel Economy Explained
The Chevy 6.6 Gas engine (GM’s L8T V8) typically gets 10-13 MPG city, 14-16 MPG highway unladen, and 8-9 MPG while towing. Owner-tracked data from Fuelly across 360 fuel-ups and 126,000+ miles on the 2025 Silverado 2500HD puts the real combined average at roughly 13.2 MPG. This is a 3/4-ton and 1-ton heavy-duty truck engine, not a fuel-economy engine — so those numbers matter most if you’re deciding between the 6.6L gas V8 and the Duramax diesel, or trying to figure out whether your own truck’s mileage is normal.
This guide breaks down real-world MPG by driving condition, what changed when GM swapped in the 10-speed Allison transmission for 2024+ models, how the gas engine actually compares to the diesel L5P, and what genuinely moves the needle on fuel economy — versus upgrades that mostly just sound good.
Quick Answer
The Chevy 6.6 Gas (L8T V8) engine averages 10-13 MPG city, 14-16 MPG highway, and drops to 8-9 MPG towing. Real owner-tracked data (Fuelly, 360+ fuel-ups) puts the combined average around 13.2 MPG. The 2024+ 10-speed Allison transmission improves highway MPG over the older 6-speed. For comparison, the Duramax L5P diesel gets roughly 19 MPG highway versus the gas engine’s 14 MPG.
Real-World Chevy 6.6 Gas MPG by Driving Condition
GM’s 6.6L L8T V8 gas engine is SAE-rated at 401 horsepower and 464 lb-ft of torque in the Silverado and Sierra HD lineup, running an iron block with aluminum heads, direct injection, and variable valve timing at a 10.8:1 compression ratio. That’s a lot of engine to move a lot of truck, and fuel economy reflects it.
Owner forums (Silverado/Sierra HD forums, GM-Trucks.com) and Fuelly’s aggregated fuel-log data show a consistent range once you separate unladen driving from towing:
| Driving Condition | Typical MPG | Notes |
|---|---|---|
| City / stop-and-go | 10-13 | Lower end with a heavy foot or cold starts |
| Highway, unladen (65-70 mph) | 14-16 | 2024+ 10-speed models trend toward the higher end |
| Towing (5,000-11,000 lb trailer) | 8-9 | Expect a 30%+ drop from unladen highway MPG |
| Combined average (Fuelly, 2025 Silverado 2500HD) | ~13.2 | 360 fuel-ups, 126,000+ tracked miles |
That combined 13.2 MPG figure is worth trusting more than any single owner’s story, since it’s an aggregate across hundreds of fuel-ups rather than one truck’s best (or worst) tank.
2020-2023 vs. 2024+ 10-Speed Allison: Does the New Transmission Improve MPG?
Not every L8T-equipped truck gets the same real-world mileage, and the transmission is the biggest reason why. 2020-2023 model year 2500HD/3500HD trucks paired the 6.6 Gas engine with a 6-speed automatic, which tends to sit at higher RPM on the highway and lands closer to 12-14 MPG unladen. Starting with the 2024 model year, GM switched to a 10-speed Allison transmission shared with the diesel lineup — more gear ratios let the engine cruise at lower RPM at highway speed, which is the main reason 2024+ owners report highway numbers closer to the 15-16 MPG end of the range instead of 12-14.
If you’re comparing a used 2020-2023 truck against a newer 2024+ one purely for fuel economy, the transmission generation matters more than trim level or options.
Tips for Maximizing Fuel Efficiency
Regular maintenance checks are essential for maximizing fuel efficiency. Schedule oil changes and filter replacements on the manufacturer’s interval, and check tire pressure monthly — underinflated tires alone can cost you a noticeable chunk of MPG on a truck this heavy. Clean air filters help the engine breathe, which matters more on a naturally aspirated V8 like the L8T than it would on a turbocharged engine.
Driving habits move the needle just as much as maintenance. Avoid rapid acceleration and hard braking, hold a steady speed on the highway (cruise control helps), and cut down on idling time. None of this will turn a 6.6 Gas HD truck into an economy car, but it’s the difference between landing at the low end of these ranges and the high end.
Aftermarket Upgrades: What Actually Helps MPG (and What’s Overhyped)
Performance chips, programmers, cold air intakes, and exhaust systems are commonly marketed as MPG boosters for the 6.6 Gas. In practice, expect modest gains at best — typically in the low single-digit percent range in real-world driving, not the dramatic jumps some product listings imply. A tune that’s set up for more power will usually cost you MPG, not save it; a tune specifically set for economy can help marginally on unladen highway driving.
| Upgrade Type | Realistic Effect on MPG |
|---|---|
| Performance chip/tuner (economy-tuned) | Small gains possible, but not guaranteed on every truck |
| Cold air intake | Improves airflow; MPG effect is minor on its own |
| Performance exhaust | Reduces back pressure; MPG effect is minor on its own |
| Tire pressure kept at spec | One of the few upgrades with a consistent, measurable MPG effect |
Our Pick: Monitor Your Real MPG First
ScanGauge 3 Touch Screen OBD2 Scanner – Real-Time MPG & Trip Computer
Best for: Drivers who want to see live MPG while towing or driving
Why we picked it: Shows real-time and average MPG, coolant temp, and trip data right on the dash — the fastest way to actually verify your own 6.6 Gas truck’s mileage instead of guessing from the pump.
Main drawback: Touchscreen response can lag slightly in cold weather.
View Our Pick on Amazon
 K&N NextGen Cold Air Intake (2020-2025 Chevy/GMC 2500/3500HD 6.6L)Fits the current-generation L8T specifically, unlike universal intakes.Check on Amazon → |
 ANCEL AD310 OBD2 ScannerReads and clears check-engine codes that can quietly hurt fuel economy.Check on Amazon → |
 AstroAI Heavy Duty Digital Tire Pressure Gauge (0-230 PSI)Rated to 230 PSI for HD truck tires; underinflation is a real, easily-fixed MPG drag.Check on Amazon → |
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Chevy 6.6 Gas vs. Duramax Diesel (L5P): Real Fuel Economy Comparison
The most common real-world comparison isn’t gas-vs-other-gas-V8s — it’s gas versus GM’s own Duramax L5P diesel, since both are offered in the same 2500HD/3500HD trucks. The diesel wins clearly on fuel economy, but the gas engine costs less upfront and has a simpler emissions system with no DEF to manage.
| Engine | City MPG | Highway MPG | Towing MPG |
|---|---|---|---|
| 6.6L Gas (L8T) | 10-13 | 14-16 | 8-9 |
| 6.6L Duramax Diesel (L5P) | 13-15 | ~19 | 10-12 |
That roughly 5 MPG highway gap adds up fast on a truck that covers serious miles, which is why heavy towing/long-haul buyers tend to pay the diesel premium — while worksite and lighter-duty buyers often find the gas engine’s lower purchase price and maintenance costs make more sense.
Real data, not anecdotes: Fuelly’s aggregated fuel-log data for the 2025 Chevy Silverado 2500HD — 360 fuel-ups and over 126,000 tracked miles across many owners — puts the combined average at approximately 13.2 MPG, with individual 6.6 Gas owners generally landing between 11.8 and 13.6 MPG depending on driving mix and load. That’s a far more reliable picture than any single owner’s “I got 20 MPG” story, which usually reflects one easy tank rather than a sustained average.
Expert Recommendations for Maintaining MPG
Regular maintenance keeps the 6.6 Gas running efficiently. Change the oil on the manufacturer’s recommended interval, inspect and replace air filters as needed, and check tire pressure monthly — underinflated tires on a truck this heavy create real, measurable rolling resistance. Using the fuel grade specified in your owner’s manual (rather than upgrading to premium in hopes of an MPG boost) is enough for this engine; GM does not require premium fuel for the L8T.
Note: this article relies on aggregated owner fuel-log data (Fuelly) and forum-reported real-world testing rather than an in-house dyno or fuel test, since we have not run a controlled MPG test on this specific engine ourselves.
If you’re cross-shopping HD trucks for towing, see how much a Silverado 1500 can actually pull in our guide to towing a 5th wheel with a Chevy Silverado 1500, and if your truck uses the 10-speed Allison or an older Hydra-Matic behind the L8T, our 4L80E transmission vehicle list covers which GM trucks pair with which transmission.
Frequently Asked Questions
How Many MPG Does a 6.6 Gas Get?
The 6.6L L8T gas engine typically gets 10-13 MPG city and 14-16 MPG highway unladen, dropping to 8-9 MPG while towing. Fuelly’s aggregated owner data puts the real combined average around 13.2 MPG.
What Is the Gas Mileage on a Chevy 2500 6.6L V8?
A Chevy Silverado 2500HD with the 6.6L V8 gas engine typically gets 10-13 MPG city and 14-16 MPG highway, similar to the Sierra 2500HD, since both share the same L8T engine and drivetrain options.
How Many MPG Does a Chevy 2500 Gas Get Compared to the Diesel?
The 6.6L gas engine gets roughly 5 MPG less on the highway than the 6.6L Duramax diesel (L5P) in the same truck — about 14-16 MPG for gas versus roughly 19 MPG for diesel — but costs less to buy and has no DEF system to maintain.
Does the 2024+ 10-Speed Allison Improve 6.6 Gas MPG?
Yes. 2020-2023 model year trucks paired the L8T engine with a 6-speed automatic, while 2024+ models switched to a 10-speed Allison. The extra gears let the engine run at lower RPM on the highway, which owners report as a real, if modest, MPG improvement over the 6-speed.
How Much Does Towing Affect Chevy 6.6 Gas MPG?
Towing typically cuts MPG by 30% or more. A truck that gets 14-16 MPG unladen on the highway commonly drops to 8-9 MPG towing a 5,000-11,000 lb trailer, depending on trailer aerodynamics and terrain.
Do Performance Chips Really Improve 6.6 Gas MPG?
Expect modest gains at best, not the dramatic improvements some listings advertise. A tune set up for more power will typically cost you MPG rather than save it; only an economy-specific tune offers a realistic, small MPG benefit.
The Bottom Line
The Chevy 6.6 Gas engine is built for heavy-duty work, not fuel economy — 10-13 MPG city, 14-16 MPG highway, and 8-9 MPG towing is the realistic range, with a combined owner-tracked average around 13.2 MPG. If you tow frequently or drive long highway miles, the diesel L5P’s roughly 5 MPG highway advantage may be worth its higher purchase price. If you tow occasionally and want to avoid diesel emissions systems and DEF refills, the 6.6 Gas is a straightforward, capable choice — just don’t expect chips and intakes to change those numbers dramatically.
