Only 4.5-5 mpg? :(

you should be getting better mpg than that, the roof spoiler alone isnt going to cause that problem... but your also leaving out
alot of infomation to help you.. HP, Transmission, rear ends, tire size... have you done any work to the truck?

Also did this just start or has it been an ongoing problem?

My Cat does 6,2 all day long in the summer, once the winter blend fuel comes out i drop to 4.9 to 5.2

 

Check CAC for leaks.
Run the overhead.

What kind of specs and what speed are we talking.

Wait a minute. 2004. EGR or pre EGR engine?

 

I had a 2006 flat top Century with a 450 Mercedes. I had a full roof fairing out back frozen in the ground when I switched back to hauling vans. Come spring when the ice went away, I put the roof fairing back on and saw an immediate 0.7 mpg improvement. Mine averaged 6.8 mpg after that.

 

Still as mentioned twice. No advice can be given without knowing specs. What transmission, what gear ratio, what tire size and what speed do you usually run at?

Not sure about the century but my Columbia's gear ratio can be found behind the panel under passenger side fuse box on the dash (has a little columbia sticker onnit)

 

Does nobody realize that winter blend fuels are not as efficient, its normal to lose around 1mpg per gallon in late fall when we start using winter blend fuels and in spring we gain around 1 mpg whe we switch vack to summer blends....yet theres always some bafoon that ckaims its from that bottle of magic dust