6.6L gasser MPG with new 10 speed Allison tranmission?

Jim-GA · September 30, 2023

15 hours ago, Pryme said:

Here is a freeway run today 65-70 in my 6spd truck. 18.8mpg

Have you verified those numbers against the old fashioned way of calculating MPG on a tank by tank basis (miles since last fill up, divided by gallons to refill tank)? Because when I do it the "old way", I find Chevy's computer reporting MPG is a whopping 11% higher than actual MPG. This is consistent over many tanks, not just a difference in how full the tank got this time, compared to the last time. YMMV.

Pryme · September 30, 2023

5 hours ago, Jim-GA said:

Have you verified those numbers against the old fashioned way of calculating MPG on a tank by tank basis (miles since last fill up, divided by gallons to refill tank)? Because when I do it the "old way", I find Chevy's computer reporting MPG is a whopping 11% higher than actual MPG. This is consistent over many tanks, not just a difference in how full the tank got this time, compared to the last time. YMMV.

That’s off the scangauge. It’s been pretty accurate.

UWSkier · September 30, 2023

I did a freeway run this morning in my truck too. 10 speed 6.6, speeds 75 to 80 mph. Best 25 mile stretch was 19.3 but I got 18.4 over the 160 mile round trip, verified at the pump. It was my first unloaded highway run since it's been broken in, and the last 50 miles was in a 20 mph crosswind. I'm VERY happy with that.

Edited September 30, 2023 by UWSkier

Jim-GA · October 1, 2023

17 hours ago, Pryme said:

That’s off the scangauge. It’s been pretty accurate.

The scangauge is reading the data off of the GM computer. If the truck's computer is not capturing it correctly when it reports the MPG on the truck's display, won't the scan tool just be pulling the same faulty data?

I was kind of shocked at the 11% error on my truck's computer. Maybe it's a bug just in the code for the year and engine I have. And, of course, it's high, making the trucks fuel economy appear better to the owner than it actually is. This, to me, is approaching fraud by GM.

Dealer says there is nothing they can do to calibrate it. With all the electronic stuff crammed in the trucks today, everything with the dealer is a big ¯\_(ツ)_/¯ . How convenient. I wonder if a class action lawsuit would wake someone up at GM to figure out how to fix it...

64BAwagon · October 1, 2023

The mileage accuracy is far from a conspiracy (although I enjoy a good conspiracy) its just an estimate and shouldnt be taken as gospel. Its a helpful tool for seeing what different driving styles can do to effect MPG but thats about it. The only really accurate measurement is miles traveled divided by gallons burned (being careful to fill the tank the same way each time. I think your expectations for the system are unreasonable.

UWSkier · October 1, 2023

On my 4000+ mile cross-country towing trek with the camper, I cross checked the computer's calculation with the pump math and it was usually only .1 to .3 MPG optimistic. It wasn't a large percentage in my case.

Jettech1 · October 2, 2023

2 hours ago, UWSkier said:

On my 4000+ mile cross-country towing trek with the camper, I cross checked the computer's calculation with the pump math and it was usually only .1 to .3 MPG optimistic. It wasn't a large percentage in my case.

Mine too, and that's why I stopped doing the actual pump calculations. My truck was so close it wasn't even worth the extra effort.

Pryme · October 2, 2023

I usually don’t even fill up all the way unless I’m going on a trip because it takes so darn long haha I usually just throw like 40 in and head out. So it’s not often I would do the hand calculation method.

Doodle · October 2, 2023

14 hours ago, UWSkier said:

On my 4000+ mile cross-country towing trek with the camper, I cross checked the computer's calculation with the pump math and it was usually only .1 to .3 MPG optimistic. It wasn't a large percentage in my case.

My 24 is very accurate as well, Best computed average I’ve ever gotten with a GM product. I’m at 5000 miles now and was hoping unladen mileage would come up. Best I can get pure highway is maybe 16.5-17. My lifetime average is 12.7 so that’s not bad with a mix I guess. My 15 LML only gets about 14.5 average so with the cost of fuel it’s a wash. My 24 gas AT4 CCSB weights almost 7600 lbs empty. My LML SLT CCSB weighs 7850. These new pickups weigh quite a bit more and they sit up so high in the air, mileage is just gonna suffer some. For reference, I had a 2008 LMM CCSB that only weighed 7200.

Dunn · October 7, 2023

I ran my 23 6spd back to LA from AZ and reached 19 mpg brand new....if I run it on freeway with no stop and go 16-17 consistently @ 65mph...

Had a 2020 and 2022 as well and both were same

I live near the beach which means a stop sign or light every 10ft it feels like and the truck hates it....8mpg

If you keep motor going and limit the stop and go it does very well imo...

Jim-GA · October 7, 2023

13 hours ago, Dunn said:

I ran my 23 6spd back to LA from AZ and reached 19 mpg brand new....if I run it on freeway with no stop and go 16-17 consistently @ 65mph...

Had a 2020 and 2022 as well and both were same

I live near the beach which means a stop sign or light every 10ft it feels like and the truck hates it....8mpg

If you keep motor going and limit the stop and go it does very well imo...

Is that per the truck computer? Or did you calculate it the old fashioned way with miles driven and gallons pumped in to fill it up again?

Jettech1 · October 8, 2023

8 hours ago, Jim-GA said:

Is that per the truck computer? Or did you calculate it the old fashioned way with miles driven and gallons pumped in to fill it up again?

I can say the truck computed mpg is stupid accurate. Anyhow you live in North GA. We live on the southside of Atlanta and are planning on coming up your way in December for some camping action. We can't wait!

Dunn · October 9, 2023

On 10/7/2023 at 10:44 AM, Jim-GA said:

Is that per the truck computer? Or did you calculate it the old fashioned way with miles driven and gallons pumped in to fill it up again?

I calculated and the computer is really close, at times it can off but on avg it's close enough to go by

It does take some miles on hwy to reach optimum unlike a halfton.....I just go off computer now since I've already calculated it and we'll, it's a 3/4 ton truck so whatever it is all good....already had a few halftons and they weren't enough truck so.....

Edited October 9, 2023 by Dunn

2021 HD · October 14, 2023

My 2024 with a flat bed loaded with around 500 lbs average is getting 13 MPG only around town. It only has 1200 miles so it is not even broke in yet.

Doodle · October 30, 2023

Another update on mileage. I took my 24 on a 600 mile roadtrip all highway this past week. I was fighting a little wind from time to time (I live in Kansas) but I was traveling between 69-80 depending upon if I was on the interstate or state highways. I noticed once you get above 70 mph mileage really starts to drop. At 80 I was getting about 14-14.5 roughly. At 65-70 closer to 16-17. Think my average tank was about 15 or so on that trip. Kind of disappointed in the mileage at interstate speeds as it’s holding 10th gear fine, but also realize these things sit way in the air and are heavy. On one flat stretch I put it on 60 mph cruise just to see the difference and it was closer to 18-19. Only 7K on it so far. My 23 started to get a little better around 10K so guess we’ll see. Also the first tank of fuel I ran was 89 octane. Just wanted to see if there was a difference. I guess I’d say there was no noticeable difference.