I have a 96 chevy with the catalytic converter on it. If I were to remove the cat and the muffler and just fill in the remaining exhaust with the stock size pipe, would it make a difference for mileage and power? (and how big of difference) I'm on a college budget and don't want to spend the money on a 4in system.

 

You would see a improvement on milage and a little more power but I would say you would have to put it on a dyno to actually put a number on any HP gain but with the 3:73 gear ratio you ought to get decent all around fuel milage my 94 had 4:10s and i Got 20mpg around town I havent figured out what is in my 93 yet the build sticker in the dash the half with the GU code is missing having the NV4500 in OD im running 2700RPM at 65 so highway milage will suffer

 

This is a great system at a reasonable price-the best part is that it replaces the stock down pipe that sucks:
Diamond Eye K4102A 4 in Turbo Back Single Fits 94-00 GM 2500/3500 6.5L Turbo Die

A 2 1/2" crossover pipe helps as well,often the double wall stock pipe is collapsed hurting turbo performance and exhaust flow.

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Sounds like mine-likely 4:10's.

This site will give you a breakdown based on vin #:
Comprehensive Chevrolet, Saturn, GMC, Hummer, Cadillac, Buick, Oldsmobile, Pontiac, Geo, Dodge, Plymouth, Jeep, Chrysler, Ford, Lincoln, Mercury, Mazda Truck, Mercedes VIN Decoder

 

I've seen the Diamond Eye is only around $230, but I'm in dept for college and just don't have too much spending cash. Mileage was my main concern, my uncle swaps out the 6.5 for the Cummins so he has extra stock size pipe laying around that I could get my hands on. Around town only get around 14-15 mpgs, but setting the cruise control on the highway I can get 21, (this is from recent "winter" fuel calculations). Summer time I've gotten 24 highway and 16 in town, but I live in a town that was built on a huge hill so it kinda hurts mileage, especially with a stick shift. I've heard of those 1st Cummins getting 28 so I was trying to match.
With spring break coming up, I think I'll have time to work on my pickup some.