I have a RCSB+G80+GT5=17mpg's in town and about 18mpg's highway.

 

i'm averaging 18 mpg with 3.90's, 70/17/265's Bridgestone Revos. I noticed a drop both after the wider tires and ESPECIALLY the 3.90 install, but it was nothing I couldn't handle.

 

My truck came with 3.73's and would get 16mpg Highway when it was stock... After Turbo, tuning, 4L80e trans and 33x13.50R18 tires it got 8mpg on the highway... I just put in 4.56 gears and now i am back to getting 14-16mpg on the highway!

It would seem the reason is because the 3.73's had my rpm's at 80mph right at 1,800 rpm and that is too low for highway cruising, the 4.56's put it at 2,700rpm at 80mph. Also, now the truck does not shift out of overdrive unless you really get on it, it has no need to because it can build full boost in over drive almost instantly! Some times it will just unlock the TC but, only down shift to 3rd if you get on it hard enough...

 

I've got stock tires and rims with all mods below and I have a heavy foot thats why I get 12.5 in the city.

 

if you're wanting better gas mileage then just lay off the go pedal.

 

I don't understand i get around 12.5 city (I don't know Highway cause all i drive is city)..... I do have some heavy *** 20's and 3.42 but other than that it is stock. When I bought the truck brand new it came with the 20's and i have never gotten above 13. What do you guys think the combination of the city driving, 4.8 and 20's causing my mileage to suck.....

 

I have a heavier truck with a bigger motor and get the same mileage, I got 12.8 on the last tank. Its that 4.8 IMO. If you had a bigger engine it wouldnt have to work as hard ot get those 20s rolling, therefor better mileage.

 

The key factor in determining mileage in LS1-based motors (on the freeway at least) is engine RPM. If you keep your RPM between 1800-2000 you will get optimum mileage. The 1800 number is ideal, the loss when moving to 2000 RPM is less than 1 MPG, but as soon as you start going over that you start losing mileage pretty quickly.

When going from 3.73's to 4.10's in my Avalanche, I lost 2-3 MPG on the freeway because of the increase in cruising RPM.

 

WOW.. i think i have just learned how to drive my truck perfectly...
In town (not city, Crystal springs is pretty small) but last few times i checked it i have gotten 21-22.5mpg.. thats 100% driving in town.. LS1 fans netted me about 1mpg better.
The worst i have ever gotten was 14.2mpg,(best ever 24.1mpg, after hurricane katrina.. ), that was pulling a 16' tandem axle trailer with my Honda Foreman 500 with tires and rims extra gas, clothes and a 06 Grizzly and my friends gas, clothes and bout 125lbs of tools in the bed.. We held 80mph from crystal springs to magee, and 100mph from magee to hattisburg, then driving through hattisburg, wiggins, and perkiston with Tow/Haul on. This is with 3.42s also

After driving bout 3 weeks and gettin down to 1/4 tank i went to byram and raced about 5 times and then raced my friends QC Hemi home playin on the interstate.. when i filled up i got 16.9mpg..

Im wondering what it will do when i put the 4.10s, LT headers, O/R Ypipe, 3" exhaust back to my magnaflow and dump over the axle. and a lil more tuning,
The best thing is the bed cover, they are awesome, keep stuff dry in the bed, help MPG some, and looks good too.

 

I added 20" rims, cam swap and still get about 20 mpg on the hwy and 16.5 city. Have Corsa exhaust with shroties, neslon tune and 3.73 gears (soon to be 4,10's).