Straight pipe is pretty obnoxious outside the truck but not terrible inside the truck. If you are worried about noise then without a doubt put a muffler on the truck.

I am not worried about outside noise its the inside that I don't want to be extremely loud.

Short answer is drive slower...driver plays the biggest role in mileage.

Unless you plan to keep your truck for a lot of miles (talking 200k plus), you will likely never see the return on the money invested in mods for fuel mileage. Although it does make it a lot more interesting to drive! $1000 will buy a lot of fuel versus a 1mpg increase in mileage from $1000 in mods...something to think about.

With that said, a Smarty will not likely give you a significant increase in hand calculated mileage; it will only make hitting your current mileage "high" more consistent/easier...for example, if you hit 20mpg once in a while, you will likely find it easier to hit that mark and more often. Straight piping, unless you have a DPT; will yield very little if any mileage....the stock exhaust is just not that restrictive. The biggest thing to improve mileage (other than driver input) for myself was installing a set of DynaTrac lockout hubs...that's between 0.75 and 1.5 mpg increase, and varies from person to person.

I have done the "mileage mods", and I have the records to back my statements up for the past 260k...I got a high of 21 mpg when bone stock and less than 1000 miles on the clock, and I still get pretty much 21 mpg at 260k with a SmartyJR, exhaust, hubs, and intake...and that's with 4.10s!

Its not easy to "make" something with the aerodynamics of a brick more economical, wind resistance (not programming, intake/exhaust, etc.) is your biggest hurdle in these trucks. I know this from literally miles (over 500k between two trucks) of playing around trying for that huge increase...my best hand calculated tank on this 04.5 was 26mpg (yes, 26mpg); but that was an unusual set of circumstances: 55mph on the interstate through the flat lands of North Dakota, with a 45mph tail wind...I got two tanks back to back that hand calculated at 26mpg! The return trip on that same route netted me 15mpg because I was bucking a 45mph head wind...you win some, you lose some.

Based on my experience, I have made up my mind that I will probably do only two things on my next truck...a in-bed fuel tank similar to my current setup to prefilter my fuel (to help protect the injection system) and a muffler (for nothing more than noise).

Does it matter pipe size on straight pipe?

The stock pipe is 4 inch, if that's what you're asking?

I know that im wondering more on the lines of what size i should replace that with when i cut off the cats and muffler?

Actually your current pipe size is 4", and its quite efficient (except the stock muffler, and possibly cat). Unless your going for upwards of 450 hp plus, or was it 500 hp :humm:
Anyway unless youre going for serious mods, your stock pipe size will do. Aftermarket Exhaust systems can get pricey, but that depends on what you want. I, recently changed my tail pipe section to a 5" inch from the muffler back. Its got the 5" look from the rear, but from the muffler to the Turbo its 4" AFE. No performance gain, but looks cool. :c: Freeing up the exhaust will also bring down your EGTs. Thats a good thing. :tu: