Auto Trans Difference Numbers

[tbmmbt]

Been looking at buying a new truck. Some have the 6 speed trans and some the 8 speed trans. How come on the window sticker the 6 speed gets better mpg as compared to the 8? Both have the 3.42 rear so I don't understand why the 6 gets better. On the stickers the 6 is 16 city 22 hwy and the 8 says 15 city and 20 hwy. Like I said both 3.42 and also the same 5.3. Any help would be appreciated. Thanks

 

[MrHorspwer]

Got a picture of the labels?

I just built two 2019 Silverado 1500 4wd on fueleconomy.gov, both with a 5.3L, one with an 8-speed, the other with a 6-speed.

8-speed shows 16/22
6-speed shows 15/20

 

[HangFire]

Different model years?
Different wheel size?
2WD versus 4WD?
1500 vs 2500?

 

[tbmmbt]

No. The 2018 are reverse from that.

 

[tbmmbt]

2018, everything the same. 4x4. Both are GMC SLT with the 3.42 rear end but the window sticker shows all the 8 speed trans that we looked at say 15 and 20 but all the 6 speed trans we have looked at say 16 and 22. Makes no sense. The 2019 change from truck to truck.

 

[joegreen]

What about transmission gearing. Just because both have the same rear end gearing the transmission final ratio might be different.

 

[tbmmbt]

Well all the 8 speed must be the same as well as the 6 speed because everyone of the 8 speed we looked at as well as everyone of the 6 speed says the same thing and the 6 speed has always said 16-22 with the 8 speed saying 15-20. To me it doesn't make any sense and when we asked different dealerships about it they all said the 8 gets better mileage but the window sticker contradicts their claim. I can only go by what is in the window of the different trucks we looked at. I thought someone here might be able to explain.