Need Help! - 00 GP GT Getting Poor MPG

[John1978BB]

This one has been bugging me for a while, can't figure it out. I have a 00 Grand Prix GT with 100k miles (had it since 60k, has always been the same way with gas), getting poor MPG. I took a 400 mi trip where it was 95% highway driving, and found I was getting about 18-19 MPG. The car is maintained like nuts so I'm not sure what the problem is.

Mods - Trasher CAI, MSD Coils (same MPG with stock and MSD), Delco Iridiums, ZZP Stage 1 TB (did it before and after TB), SLP Loudmouth exhaust, DHP PCM (same gas mileage before and after this), 180 T-Stat.

As for what I've done maintainence-wise/to try to fix this:
Oil every 3000 miles, Mobil 1 full sythentic
PCV every 3000 miles
Air filter every 3000 files
Fuel filter once a year (delco only)
Coils (MSD) replaced this year
TB + Sensors clean twice a year
Plugs new as of Jan 06, gapped correctly
All fluids (except power steering) changed once a year (except oil, every 3k)
UIM Replaced this year (no impact on gas mileage)
Shell 93 Octane gas used 90%+ of the time
Car is totally empty, nothing in the trunk or anything

I'm really racking my mind here. I'm thinking that 19 MPG highway is just too low for this car, considering I've heard of people getting around 30. The only things I can think of are:

Wires - seem to be okay on inspection so haven't replaced
O2 Sensors - Really one of the few things left that hasnt been repalced that can impact gas mileage
Clogged cat?

Any ideas on this?
Thanks and god bless!

[John1978BB]

Forgot to add:

I'm in NY so by highway I'm talking like 65 MPH.

Thanks again!

[richtazz]

Do you have a CEL on? A clogged cat will usually hinder performance, but cannot be ruled out without an exhaust back pressure test. The wires can be ohm tested and shouldn't exceed 500 ohms per inch of length. The oxygen sensors are probably due for replacement, especially with your mods. My 99 GT, although unmodded, gets 32MPG on a trip, and better than 20 around town (and I'm no lightfooted driver either) so there is definitely something wrong. Hopefully someone more experience with your mods will have a better answer for you. I'd start with testing/changing the plug wires and changing the oxygen sensors and see what happens.

[John1978BB]

No CEL lights or any current known problems with the car. I'll see about that backpressure test and change the wires and O-2 sensors.

Are the O2 sensors a DIY? Any certain brand I should use (I'm assuming stick with Delco?)

Thanks!!

[BNaylor]

I'm surprised he's not using a hi-flow CAT convertor. The Magnaflow are popular and hard to beat the cost.

I agree with Rich, check into the 02 sensors mainly Bank 1, Sensor 1 (before CAT) and get the CAT tested for efficiency.

And use the AC Delco brand 02 sensor only. I would not recommend using aftermarket such as Bosch.

[richtazz]

Yes, the o2's are DIY, and only use Delco. Anything else (especially Bosch) and you're taking a crapshoot. The downstream sensor is a PITA, especially if you still have the U-bend, but it isn't impossible. It's in a real cramped spot and getting a tool on it is the PITA part of it. BTW, I forgot to welcome you to AF (how rude of me ), I see you're relatively new here.

[John1978BB]

Thanks for all the advice and the welcome

One last question:

Am I correct in saying that outside of the wires, O2 sensors, and CAT, there's really not that much more there is to replace that would have an effect on gas mileage?

Also didn't mention in the first post: recent alignment and all tires properly inflated.

Thanks again!!

[richtazz]

The only other thing I can think of is excessive weight in the driver's right shoe, .

[John1978BB]

I'm innocent to that

I'll get to the O2 sensors and wires this weekend, thanks again for all the help.