^ +1 on maybe the precat O2 got damaged. you got any black smoke indicating rich fuel condition.

i wouldn't think the cat is directional...

 

all cats I've seen have been straight through honeycomb design, so I wouldn't think that would matter either.....but never know

 

Got to thinking about it...What you have basicly done is created a dead zone in your exhaust by installing a cat with 3 in diameter tubes. Since your truck is pretty much stock the exhaust you had produced velocity which carried away engine exhaust gasses. By adding the bigger air space (cat) you slowed that velocity. You then restricted the exhaust flow, thus creating back pressure, by forcing it back into the smaller diameter tube. That is the reason you see exhausts that flow into a 3 in. input of a muffler exiting and continuing with the 3 in. tube or dual 2.25/2.5 tubes until termination. Sorry if Im not clear. Back pressure is a farse with todays computer controlled timing and fuel injection vehicles. Velocity is the key, that is why different size pipes work better on different sized engines.

 

The inlet/outlet doesn't really matter, no matter what when exhaust enters a cat it expands to a larger volume then has to be pushed back into a smaller exit again.

 

Ok I agree with most of this backpressure/velocity stuff, but I'm not seeing that too cause that big of a difference in mileage, maybe some loss of power and slighty less efficient but not 100 miles less a tank, my guess is precat 02 sensor and thats all that I can think of unless its a coincidence something else went like maybe plenum, any pinging??

 

I do not disagree other problems are to blame. I am just trying to explain how it could be the cats design that is at fault. For example the Honda guys install these large mufflers and some find that they lose performance. Upon reinstalling the muffler it restored performance even gaining if replaced with another aftermarket. When you lose performance you tend to increase fuel consumption to make up for it. Who's to say it is not a combination of faults? A suggestion was made to reinstall the old cat, good idea.