Bye the way, magnaflow does offer a cat-back system for our truck. Just use the systems for the ss and it will bolt right up. Part # 15838 is a single and #15841 is a dual system.

 

If I keep my foot in it I average a little less than 16mpg. If I behave myself I gat a little better than 17. Straight highway driving at around 70 is right at 19 mpg. The best I ever got was at 65mph, a/c off, tucked in behind 18 wheelers. Mileage was around 20.5 mpg on that trip. Yes, the VHOs have a 3.73 gear and 31" tires. My mileage has remained basically the same with my mods. They are intake, Flo-Pro muffler and tune.

A couple of things to consider on the mpg loss. A/C effects mileage, as does wind and degree of road incline which may not be easily noticeable.

 

4.10'S ARE GREAT IN MY TRUCK. I LOVE THE WAY I LIKES TO RUN THROUGH THEM! ON THE HIGHWAY THEY DO FINE AS WELL. BUT, THEY DO SEEM TO MAKE MY DENALI THIRSTY. dIDN'T REALIZE THE VHO DIDN'T HAVE THEM. OOPS! WITH GAS PRICES RISING AND MY LS1 ONLY DRINKING 93 OCTANE I MAY BE CONSIDERING A GEAR CHANGE AS WELL.

 

I forgot to mention that most of my driving is bumper to bumper driving in Dallas.

 

The best my VHO will accomplish is 15.5 mpg with AC off and cruising at a highway speed of 65 mph.
I just cannot see you guys getting 17 mpg and higher in this particular truck.
I sure have not seen it crack over 16.5 mpg yet.
I assure you that I've been driving it easy.
I'm not impressed with mine against all of you with VHO's.
I forget who said that the 04 SS exhaust will bolt up to a VHO, but I tend to differ on that one.
I had my local GM parts manager pull up the factory cat back exhaust system part #'s and pictures of both the SS and VHO. They are totally different.
At least compared to my 05' VHO.
Corsa even said the 04 SS exhaust system will not bolt up to the 05 VHO.
They will have an exhaust system for the 05 VHO's early or mid summer.

Jim

 

Corsa told me the same thing, but there is a guy with a VHO on this site running a Corsa exhaust! I forget his name. I also believe the SS exhaust and the VHO exhaust are one in the same with the exception of the nice tips that the SS exhaust has. Maybe that is the difference for the part numbers.

I forgot to mention earlier that my mileage numbers were with 35psi of tire pressure on mostly flat roads. I am seeing wear in the center of my tires and have backed the pressure down. I assume mileage will be somewhat less, maybe not much. We'll see.

Instead of buying a entire aftermarket exhaust, I simply bought a higher flowing muffler. There are many available out there. I went with the Flo-Pro HD. The stock pipes design from the cats back is not bad at all. I can't see where there are any restrictions that would hamper performance. It is basically a personal choice. I want to look as stock as possible and spent the money I saved elsewhere.

How many miles do you have on the truck?

 

I have Corsa on my '04 VHO. The stop and go driving really hurts my MPG. Are you getting your number from the cluster or at the fuel pump?

 

I avg 15.5 city. HWY usually gets around 17 but I drive 75-80. I have had a few 20+ MPG tanks. Every one of those tanks were in the mountains. I think the hills really help these trucks MPG. Going up hills my MPG drops to around 8 but going down it would hit between 40 and 85mpg. I think hills hurting MPG is an urban myth bucause what goes up must come down.

 

The LQ9 has a lot of torque, which helps it get up those hills.

 

i agree, dont worry about the whole cat back setup. just go buy a magnaflow or corsa muffler. that would be just fine. and random technologies i believe sells the highest flowing cats on the market. you could always gut your stock cats though too, and save about 350 dollars.