My last fill-up I averaged 12.84mpg with mostly city driving. Damn near everything is new. The engine has about 6K on it so it should be broke in by now. The trans has about 3K on it. I do have the lower gears, but it wasnt that big of a drop. I started with 3.06 and went to a 3.33. The engine was completely rebuilt and runs real strong. It has a new cat, O2, K&N, Syn. oil. It has a custom chip which corrected the trans shift points so it goes into OD and TCC lockup correctly. I turn about 1900 rpm @ 60mph. It doesnt seem to matter how I drive, my mileage stays about the same. I was getting about 16 mpg on the highway. Any ideas???

I used a Denso O2. But it was just as bad before I changed that and before the chip. I didnt change anything with the fuel except to allow more spark advance. Would the gears make that big of a difference???

I don't know a lot about motors, I admit that.

So here'* my shot in the dark. Is it possible that the P&P could have messed up the A/F ratio or something like that.

And this may seem stupid but is all connections hooked up, not loose?

Trying to help out here.

what do your plugs look like. I would expect them to be black, running pig rich. They should give us a clue anyway.

My first guess woudl be your O2 sensor. Do you have a way to have it tested with a wide band O2? That would give you an idea as to what your ratio is.

I checked the front plugs around the time I put the tranny in. They were an almost perfect tan color. I plan on putting some Iridim plugs in soon so I will see what the others look like. I have a snap-on scanner at work. The last time I scanned it my O2 was reading correct. All my data looked good on the scanner. There are no vacuum leaks, all emission systems are on and working, and the motor runs real strong. Im stumped.

pcv?

Its new along with the UIM.