I have an 06 express 3500 with the 4.8 187000 km (everything is stock) and i'm getting between 7-9 mpg with 100% city travelling.

I use the truck for deliveries and I always carry between 1/2 ton up to 3 tons in the truck almost all day.

I was wondering if it was normal that i get such mileage with all that extra weight ?

 

constant stop and go is what is killing it.
My burb gets 20 highway over several 100 miles. In town it gets 12 ish

 

That is a lot of weight...jeez. Still, you may consider checking the usual things. How is your tranny shifting? If your torque converter is slipping you would get bad mileage. Also, you could have the emissions checked. A slow downstream O2 sensor could also give bad mileage. City driving with that weight.....ouch.

 

The tranny shifts really well. I can hardly feel it when it shifts in second and i don't feel it at all when it shifts in third.

 

That much weight with the 4.8... yep. My 3500 gets 11 or 12 loaded up with the 5.7, so Im guessing the smaller engine works harder. And that's rural driving.

 

What is your axle ratio? Mine is 3.73, and I have the voertec 350. I get 13 in town, and 16 hwy. I do mixed hwy city as a tradesman so I have tools and materials in there....

 

How can I know my ratio ?

 

Check your option codes on the sticker. Or you could if all else fails mark a chalk line on the driveshaft and count its rotations fore one complete wheel rotation.

 

Just found out i had a locked 3.73 differential

 

Your diff is the same as mine. The eaton g80 locking differential. Its a good one, but don't spin the wheels in the snow or you will break the locking mechanism. Mine has a few broken teeth, and now is just a crappy one tire fire.
I have a 3/4 ton, and the 5.7 vortec. I get 13 in town, and 12 pulling a boat. Hwy I get about 15 either way.....You must be getting poor mileage due to a few things.
You have the smallest v8 so its working beyond its sweet spot for power and effieciency...
You are pulling a lot of weight in stop and go traffic....
You are running a 1 ton chassis, so you have more curb weight to start with....
You are not tuned up. Bad o2 sensors, bad emiisions, bad something.
All or some of the above.
Keep this in mind.
My dad has a 2008 chev silverado 1 ton dually. It has the duramax diesel and the allison tranny. He gets 12 on the hwy, and 9 pulling the stock trailer.
No too different.....