My LQ4 powered 1500HD ( If you had a 3.73 geared 2500 you had a light duty with the 9.5" 14 bolt which means the same truck as mine) when stock could get 15 mpg US fairly easily on a highway run.... That is with a 4L80E and 3.73 gearing. Stock height, stock 245/75R16 all season rubber. I get around 12-13 mpg US now with 245 DuraTracs, a Pack Rat roll out and Weatherguard tool box both loaded with tools and gear and a headache rack. I should add that I have added approximately 1500 lbs to my trucks weight in the process here too. Needless to say there was something wrong with your previous truck, IMHO.

A lighter truck with an LQ4 tuned properly and some sane driving will get better than your Gen 1 dinasaur 350..... and outrun it in every single way shape and form. But that's just a given, lol.

 

my Stock LQ4 would get about 17-18 on the hwywith 3.73 and a 31" tire. But Im only getting about 13-14 now lol.

 

well ya'll definately gave me more to think about. wish i could afford a 6l80 to go with the new motor but that's going to have to wait. where's everybody finding the awesome motor deals i read about? i'm been looking aorund on car-part and a few other salvage sites but haven't seen anything like some of the deals i've read in swap threads

 

6l80 will cost you mileage. Do a manual swap and enjoy having control.

 

how will a 6l80 cost me mileage? i thought about swapping in a nv 4500 since i miss having a standard just not sure i want to deal with it in traffic

 

Guess it depends on where you live-I enjoy a manual most all the time. Although a 6l80 is better than most, you'll always have more driveline losses with an automatic vs a manual. If it was a 2wd, Id do a T-56.

EDIT: I just went back and checked, and you dont say whether it is 2wd or 4.....

 

The 6L80E shouldn't cost you any mileage vs your current setup, as the overdrives are nearly identical and the difference in drivetrain loss is minimal. However, if you were at all considering a manual swap, then yes, a manual would be more fuel efficient.

 

By current setup, I mean a 4L60E. The 6L80E is not noticeably less efficient than a 4L60E, as it seems to more than make up the difference in drivetrain loss (if any) with its better ratios.

 

the truck is a 2wd. sorry don't know why i didn't include that. would a t56 even stand up to a truck's weight and towing fairly regularly? i'm still thinking about sticking with an auto. i do enjoy a standard but dealing with san antonio or corpus traffic just makings me want to stay with an auto

 

A four-speed will be a lot easier for you, as many people have already done LS swaps in GMT400 trucks with their factory trans or a 4L80E, while the 6L80E, if it's even been done, will be more research-intensive due its relative lack of popularity.