14mpg is pretty good in the city in my experience. That's about what I got with the 4.8. Your mileage is actually higher than what you're calculating because of the taller tires

 

if ur on the interstate if u go 70mph slow down to 65mph they had that on the news a while back around here.. i know when i get off work at 2:30 in morning since there is no traffic mite meet one car once i get to the top of the mountain the speed limit is 65 and i usually set cruise at 50 or 55 lol

 

I do that as well. usually 55 or 60.

 

yup it helps out more than youd think checked minea while back and was getting 19.2mph

 

get rid of the 22's and it'll make a big difference. then make sure you are calculating the difference in tire size also.

 

go ahead and get a custom tune, then report back and let us know what kinda mileage increase you gained..

 

You can't just say a specific MPH to run at it depends on your effective gear ratio. I believe for optimal fuel economy the LS should be run between 1500 and 1800 rpm. For me that's between 60 and 70mph. I've netted over 23mpg with the 6.0 WITH my original 4.8 in the bed lol doing about 65 cruise over a 400 mile trip.

 

I know what your talking about... but most of the time when you hit a hill or something its goin to bog and downshift.. ive always "heard" that 1900 is the sweet spot for LS's..

im turning 2150 at 70 with the 80E/3.42/28"
and it does pretty good..
i cant remember what my rpms are at 55 but ive hit some nice size hills and it didnt down shift or bog and i think i got into boost a time or two..

 

actually, the very reason you mentioned is why I propose the 6.0 gets better highway mileage than the 4.8L. The most noticeable aspect to me of the LQ9 when I was first driving home after the swap was how it absolutely powered up (big) hills without shifting out of overdrive. By comparison, the 4.8 was revving to the 2500-3000 range to go up the hill. I think it's all a pretty tough balancing act. These LS motors make a good amount more torque at 2000rpm than they do at 1500. So if your gear ratio dictates that you will do 2000rpm @ 65 mph then it's unlikely you will have to downshift going up a hill, but you're also out of the efficiency range. The key I think is to have enough torque to power up hills in OD around 1600rpm without downshifting and sucking gas. This is where the 6.0 comes in.

 

I drove 1300 miles this weekend. Saturday we went to TN/KY and on the way up I got 12.5 on the highway. On the way back 11.7 granted I had some stuff in the bed. Several hundred pounds. Yesterday I went above Atlanta towing a Corvette that belongs to a friend. I got like 9.9 towing the trailer. That was running 75-80 the whole way on all trips. At that MPH it is turning close to 2500 rpm and rarely downshifted going up and down the hills. Any faster it would downshift to keep speed. That speed seemed to be the best speed to run.