Ever since I started making changes to my 01, my fuel millage has been going down. When I changed my plugs and wires I went from 16 avg mpg to 14.5 avg mpg, and now when I put a smaller pulley on, I'm lucky if I get 13 even driving without the lead foot. However when I do drive with that lead foot I get about 10.5. What the heck. I don't mind paying the price for gas but I don't want to have to fill up just to go around the block. I was perfectly fine with 14.5 and even 13.5 but 10.5 come onnnnn!

I/O, what about your O2 sensor, has it been replaced recently? Fuel filter replacement? Excessive transmission slip? There'* a few ideas to check into that could possibly be degrading your fuel usage.

Agreed, you may be seeing your mpg drop due to clogged cat or fuel filter or bad 02 sensor. Even with your mods you should see 25+ mpg cruising.

Thanks I will check both of these this weekend.

Sorry to jump in but is there any other way to check the cat other than banging it for rattle?

Just about any shop should be able to do a backpressure test to see if it is getting clogged.

You can use a low pressure fuel pressure/vacuum gauge to test the back pressure by fitting the rubber hose to a modified oxygen sensor. You need an old oxygen sensor with the same threads as your car. Cut the metal above the threads, and tear the porcelain guts out so you are left with something that looks like an empty spark plug. Fit in a brass hose barb or other pipe you can hook to the tube running to the gauge. Use hi -temp silicone if needed to make sure your pipe doesn't leak.

Run your car to normal operating temperature. Disconnect theO2 sensor wiring; unscrew the O2 sensor from your car, and replace with your pressure tap. Fire up the car and see what the pressure is at a variety of throttle openings. For my '95, pressure above 1.25 psi at 2500 rpm indicates a restriction.

IO, I don't see any mods in your sig that should degrade mileage significantly at all. In a apples to oranges comparison, my 93 is far more heavily modded, yet I still get 29+ on I-5 trips. This is over the mountains, too.

The 00+ is heavier to begin with, so you have a slight disadvantage there, but also the CA ethanol blended 91 octane fuel is probably costing you at least 5%. You're still way too low even taking these two factors into account. I agree that you probably have a front O2 sensor or Cat issue. With your mods, a high-flow cat would be a good thing anyway.

I don't know, but one would think that this would not happen just after new mod'*. But I just do the change this week end have a few other thinks to work on as well. Getting my replacement PCM from intense on friday, so it'll be a Car, Beer, and NASCAR weekend.