6.6 Fuel Requirements

bill3232 · July 15, 2022

I towed a large horse trailer and my truck both loaded with our horse, 14 heavy bales of hey, 20 gal water, tools, etc. (app. 8000 lbs.+) on 89. The truck averaged 10.8 GPM for 987 miles from South Carolina to Conn. thru the mountains of Virginia and Penn. Power was no problem at all, ran great.

MacLaren · August 9, 2022

I may try mid or premium grade while towing, but all I've ran is 87 and my mileage has been between 16-18 while not towing. This is gotta be far & away the best 2500 gasser ever built. Not just Chevy but, any. All things considered.

64BAwagon · December 3, 2022

I tow a 12,000# plus fifth wheel with mine. Never heard a single complaint from the engine, it seems to like 87 just fine.

Jettech1 · December 4, 2022

I only tried 93 octane once and didn't notice any difference in mpg nor power. I was hoping the ECM would advance timing and make it worth the extra money for the 93 in power and mpg. Nope...nothing....didn't notice any difference at all. I had high hopes though....87 for me.

sheath · December 4, 2022

17 hours ago, Jettech1 said:

I only tried 93 octane once and didn't notice any difference in mpg nor power. I was hoping the ECM would advance timing and make it worth the extra money for the 93 in power and mpg. Nope...nothing....didn't notice any difference at all. I had high hopes though....87 for me.

Same results with 91 octane (high altitude) pulling 15K. No discernible change in fuel mileage, no noticeable difference in power, longer time to start when temp was below freezing.

Following Owner's Manual recommendation has worked out well.

Since this truck has direct injection, fuel quality from a station with a high turn over rate is our focus. Thankfully that is pretty easy to find around here.

CamGTP · December 5, 2022

On 12/3/2022 at 8:31 PM, Jettech1 said:

I only tried 93 octane once and didn't notice any difference in mpg nor power. I was hoping the ECM would advance timing and make it worth the extra money for the 93 in power and mpg. Nope...nothing....didn't notice any difference at all. I had high hopes though....87 for me.

Probably the only time 91+ octane would help anything would be if it was 90-100+ degrees outside and you are pulling a heavy load up many steep grades. The higher octane would have a better chance at keeping knock away than 87 octane. So it would keep it's full power then. GM does torture test these trucks this way with 87 octane, so you know it can handle the abuse and not hurt the engine at all.

Doodle · December 7, 2022

Been running E15 (88 octane) in my 2023 since new. So far only about 2500 miles and seems to be around 14.5-15 highway. Only towed heavy once so far (24K GCW) in strong winds and maybe got 7-8. I know it’s probably not quite broke in yet but I was hoping to maybe see 16-17 like some have reported but so far 15 is the best I have seen all highway. Lifetime mileage is 12.7 so if it can stay around 13 I’ll be happy. My 2015 LML lifetime mileage is 14.7 with 120K. So with the price of fuel the 6.6 gas is quite a bit more economical right now. After i get another few thousand miles on might update the mileage. One of these days I’ll do straight gas and see what the mileage is. In my area the difference between straight gas and E15 is like $0.70/gal so it’s doubtful it would be cheaper to run even with better mileage. E85 is only $2.20 but mine is not Flex Fuel.

Pryme · December 8, 2022

9 hours ago, Doodle said:

Been running E15 (88 octane) in my 2023 since new. So far only about 2500 miles and seems to be around 14.5-15 highway. Only towed heavy once so far (24K GCW) in strong winds and maybe got 7-8. I know it’s probably not quite broke in yet but I was hoping to maybe see 16-17 like some have reported but so far 15 is the best I have seen all highway. Lifetime mileage is 12.7 so if it can stay around 13 I’ll be happy. My 2015 LML lifetime mileage is 14.7 with 120K. So with the price of fuel the 6.6 gas is quite a bit more economical right now. After i get another few thousand miles on might update the mileage. One of these days I’ll do straight gas and see what the mileage is. In my area the difference between straight gas and E15 is like $0.70/gal so it’s doubtful it would be cheaper to run even with better mileage. E85 is only $2.20 but mine is not Flex Fuel.

You would be getting that 16-17 hwy unloaded with 87oct. E10. The 88 won’t get as good at 87. But it’s cheaper. Could be a wash but the ethanol isn’t that great for it anyway.

CamGTP · December 8, 2022

The E15 does no more harm than E10 fuel, that's why it's safe to run in basically any vehicle built within the last 20 years. Fuel lines are hard plastic or metal nowadays so it can't hurt those either.

These trucks already account for the stoich being 14.1 for E10 fuel and E15 is really never 15% alcohol either, it's more like 12-13% everywhere I find it and E10 fuels is more like 7-8%. I agree that E10 will net better mileage but it's not doing any harm running it.

From some crude math, the E15 would need to be probably 20 cents cheaper per gallon to make it be a small gain but still nearly a wash. I run it my car but I drive all city and it's garbage fuel mileage no matter what fuel I run, it gets the same no matter what. And even on the freeway it still gets 30mpg, so I don't care.

Doodle · December 9, 2022

I’m gonna run some straight gas one of these days and see if there is much of a difference. I bet it won’t be any better economically. Going from 15 to 17 mpg and $0.50 (here locally) would likely be more expensive to run the straight gas.