The gas mileage is slowly rising again. I have notice with this car when ever I change something the as mileage seems to stink for about 3 days. I am planning to get another oxegen sensor as soon as I have the money. I do not think that is my problem, however I do not like that fact that its a bosch. I had 248 miles on half tank this week. Thats not bad.

I can usually get a little over 300 miles on a half tank, and the top mileage I ever got was 26 mpg

Now that I opened up the air intake some, as soon as I am able to get a better exhaust system going that should improve my gas mileage too. I have a brand new O2 in but its a bosch and I am not impressed with its performance much so as soon as I am able I will be replacing it with a delco

Ok, there'* alot of things in play here. Your air is entering the front of the box on one side. It then goes through a square filter, and exits the airbox to the intake plenum on the other side. That'* restriction, but it'* flowing better than stock. The biggest problem is that the K&N filter will get dirty right behind where the air comes in. Rotation of the filter (180° flip) would be a good idea come springtime. As the filter gets dirty there, it has to go to a different part of the filter to find a way through. Path of least resistance.

Sometimes it can take up to 100 miles for your PCM to re-learn after a major change. In your case, with a suspect O2, your 'learning' symptoms could have been worse. The PCM is adapting to changed inputs from your MAF and IAT right now. Just give it time. If you don't get at LEAST 26 highway, change that O2.

Your fuel curves are all 'up in the air' right now as far as the PCM is concerned. To be safe, disconnect your battery for 30 minutes, re-connect it, and go through an entire tank of gas (without changing octane), then check for performance/efficiency.

I noticed since last fill up my mileage is down again. Shes running great. I have been driving in D all the time now and not OD. I guess that could explain some of the loss.

I don't know how that tube you took off on your Bonneville looks but on my ssei the tube does get narrower and and then it flares out on the end. Kinda like a saxaphone or a tuba. The reason it'* like that is to increase the air velocity.