Chevy 6.6 Gas MPG: No EPA Rating, Real Test Numbers

The Chevy 6.6 gas engine (GM’s L8T V8) has no official EPA MPG rating in heavy-duty Silverado trucks, so any single “official” number you see is not from the EPA. What exists is third-party testing: one tested 2020 Silverado 2500HD gas crew cab returned 16.0 MPG empty at 70 mph and just 6.9 MPG towing a 16,000 lb trailer. This page shows why there is no EPA number, the figures that can actually be traced to a named source, and what moves MPG up or down on this truck.

Earlier versions of this page listed city, highway and towing ranges and an owner-log average. We could not trace those to a source we could read, so they are gone. Everything below names where it came from, and anything we could not verify is labeled as such.

Real-World Chevy 6.6 Gas MPG by Driving Condition

Why there is no EPA number. Per fueleconomy.gov, vans, pickups and SUVs weighing more than 8,500 pounds gross vehicle weight are classified as heavy-duty vehicles, and fuel economy regulations do not apply to them, so they are not tested. The Silverado 2500HD and 3500HD are built for that class, so the window sticker has no MPG line. Check the GVWR on your own door-jamb label to confirm where your truck falls.

GM Powered Solutions describes the 6.6L L8T as a 6564 cc OHV V8 with direct injection and a 10.8:1 compression ratio, rated at 401 hp at 5,200 rpm and 464 lb-ft at 4,000 rpm, and it says peak torque is 22 percent higher than the older 6.0L HD gas engine. It is a work engine, and fuel use follows the work it is doing. Here is every real-world figure we could trace to a source we read:

Source and year	Truck and engine	Condition	Reported MPG
TFLtruck, Oct 2019	2020 Silverado 2500HD, 6.6L gas, crew cab 4×4, 6-speed automatic	Empty, 70 mph highway loop	16.0
TFLtruck, Oct 2019	Same truck	Towing 16,000 lb trailer at 70 mph, pump-verified	6.9
TFLtruck, Sep 2023 (cited result)	2020 Silverado 2500HD, 6.6L gas	Conditions not stated in the article	16.3
TFLtruck, Sep 2023	2024 Silverado HD LTZ crew cab 4×4, 6.6L gas, 10-speed	Tested empty and towing 10,600 lb on a 111-mile loop	No MPG figure on the page we read

Treat these as individual test results, not averages: they come from one outlet, a handful of trucks and specific routes. They are third-party test numbers, not EPA data. Our earlier claim of an “owner-tracked combined average” is removed because the source page could not be opened to verify it.

Why 6.6 Gas MPG Varies So Much: Factor by Effect

Two trucks with the same engine can differ by several MPG. Where a number exists from a source we read, it is shown; where it does not, the effect is described qualitatively rather than invented.

Factor	Effect on MPG	Source or note
Towing a heavy trailer	Very large: 16.0 to 6.9 MPG (about 57 percent lower) at 70 mph with 16,000 lb	TFLtruck, 2019. Lighter trailers cost less, but we found no published figure
Highway speed	Higher speed costs more; each 5 mph over 50 mph is like paying about $0.31 more per gallon	DOE, fueleconomy.gov
Aggressive driving	About 15 to 30 percent lower at highway speeds, 10 to 40 percent in stop-and-go	DOE, fueleconomy.gov
Idling	About a quarter to half a gallon per hour, depending on engine size and air conditioning	DOE, fueleconomy.gov
Gross weight and payload	Heavier loads burn more fuel; the effect is not published by GM for this engine	Qualitative
4×2 versus 4×4, axle ratio, wheel and tire size	Real but not quantified by any source we read. GM’s 2025 trailering chart shows tow ratings change by cab, bed, drive and wheel size	GM trailering chart; MPG effect unverified
Transmission generation	The 2020 tested truck used a 6-speed; the 2024 test truck used a 10-speed. No traced test compares them directly	TFLtruck; see section below

The speed, aggressive-driving and idling figures come from the DOE and EPA fueleconomy.gov driving-habits page. They are general light-vehicle figures, so use them as direction, not as a promise for a one-ton truck.

How Far Does a Tank Go? A Simple Range Table

TFLtruck reported a 36-gallon fuel tank on the 2020 truck it tested; tank size can differ by configuration, so check yours. Multiplying that tank by the MPG figures above gives the arithmetic below (our calculation, not a tested range, and it ignores the reserve left in the tank).

Scenario	MPG used	Approx. miles on 36 gallons
Empty at 70 mph (TFLtruck result)	16.0	576
Mixed driving, assumed	12.0	432
Heavy tow at 70 mph (TFLtruck result)	6.9	248

The takeaway most pages miss: when towing near the limit, the stop-to-stop range falls to well under half of the same truck empty, so planning fuel stops with room to maneuver a trailer matters more than the tank size on paper.

Does the 10-Speed Transmission Improve 6.6 Gas MPG?

The 2020 gas truck TFLtruck tested had the updated 6-speed automatic, and the 2024 gas truck it tested had a 10-speed. Extra gears can let an engine run at lower rpm at highway speed, but the 2024 article we read gave no MPG figure for that truck, and we found no side-by-side test of the two gearboxes. So the honest answer is: possible, not proven. If MPG is the reason you are choosing between a 2020 to 2023 and a 2024 or newer truck, compare logged fuel use on the actual trucks.

Does Premium Gas Improve 6.6 Gas MPG?

Searchers ask this a lot, and the answer from the sources is no. GM Powered Solutions lists regular gasoline (87 octane) as the L8T fuel type. DOE says that using a higher octane than the manufacturer requires gives little to no benefit under normal driving and that the extra cost is typically higher than any savings. For which fuel each Silverado engine takes, including E85 and Top Tier fuel, see our dedicated guide.

Chevy 6.6 Gas vs. Duramax Diesel: What the Tests Show

The same TFLtruck 2023 article lists diesel results from the same family of trucks. These are different years, routes and loads, so they are not a head-to-head, but they show the size of the gap:

Truck	Condition	Reported MPG
2020 Silverado 2500HD, 6.6L gas	Loop, conditions not stated	16.3
2024 Silverado HD 2500, 6.6L Duramax diesel	Very windy conditions	18.1
2020 Silverado HD 2500, 6.6L Duramax diesel	Road trip	19.9
2024 Silverado HD 3500 SRW, 6.6L Duramax diesel	Towing 10,600 lb	11.5

The diesel generally returns more miles per gallon in these tests, but it costs more to buy and uses diesel exhaust fluid. If you rarely tow, the gas truck’s lower purchase price can still win on total cost.

How to Measure Your Own 6.6 Gas MPG

Since no official number exists, build your own. Fill the tank completely, reset the trip meter, drive normally, then fill completely again and divide miles driven by gallons added. Do it separately for empty driving and for towing, and repeat a few times, because a single tank can mislead. For the formula and common mistakes, see our guide to how to calculate real MPG. A trip computer such as the ScanGauge in the box above automates the same measurement and lets you test changes like tire pressure or cruising speed one at a time.

One honest warning on upgrades: we found no independent test showing that a chip, tuner, cold air intake or exhaust measurably improves L8T fuel economy, so we do not list gains for them. Steady speed, not idling and gentle driving are the changes with sourced support.

Frequently Asked Questions

How Many MPG Does a 6.6 Gas Get?

There is no EPA rating for the 6.6L L8T gas V8 in heavy-duty Silverado trucks, so there is no official number. In TFLtruck’s 2019 test of a 2020 Silverado 2500HD gas crew cab 4×4, the truck returned 16.0 MPG empty at 70 mph and 6.9 MPG towing a 16,000 lb trailer at 70 mph. Your mileage will depend on load, speed and drivetrain.

What Is the Gas Mileage on a Chevy 2500 6.6L V8?

Chevrolet does not publish an EPA MPG for the 2500HD with the 6.6L V8. The two third-party figures we could verify for the 2020 model are 16.0 MPG empty at 70 mph (TFLtruck, 2019) and 16.3 MPG on a separate loop whose conditions were not stated (TFLtruck, 2023). Towing a heavy trailer cuts that sharply.

Why Does a Chevy 2500 Not Have an EPA MPG Rating?

Vans, pickups and SUVs over 8,500 lb gross vehicle weight are classed as heavy-duty, and per the EPA and DOE site fueleconomy.gov, fuel economy regulations do not apply to them, so they are not tested. Check the GVWR on your door-jamb label.

Does Premium Gas Improve Chevy 6.6 Gas MPG?

No source we read supports it. GM Powered Solutions lists regular 87 octane as the fuel for the L8T, and DOE says higher octane than the manufacturer requires gives little to no benefit in normal driving while costing more.

How Much Does Towing Affect Chevy 6.6 Gas MPG?

In the one gas HD test we found with both numbers, 16.0 MPG empty fell to 6.9 MPG with a 16,000 lb trailer, a drop of about 57 percent. Lighter trailers will cost less, but we found no published figure for them.

How Far Can a Chevy 6.6 Gas Truck Go on a Tank?

Tank size varies by configuration. TFLtruck reported a 36-gallon tank on the 2020 truck it tested. At 16 MPG that is roughly 576 miles empty, and at 6.9 MPG towing it is roughly 248 miles, so plan fuel stops accordingly.

The Bottom Line

The Chevy 6.6 gas V8 has no EPA MPG rating in HD Silverados, because trucks over 8,500 lb GVWR are not tested. The best traced real-world data point is 16.0 MPG empty and 6.9 MPG towing 16,000 lb at 70 mph on a 2020 truck, with different loads and drivetrains moving that a lot. Run on regular 87, drive steadily, and measure your own numbers rather than trusting any single figure, including ours.

Sources named in this article: EPA and DOE fueleconomy.gov, GM Powered Solutions (L8T specs), GM 2025 Silverado 2500HD trailering chart, and TFLtruck road tests from October 2019 and September 2023. We have not run our own fuel test on this engine.