Hyundai Repair: bad gas mileage, torque converter clutch, poor fuel economy

Question
Hi,

Thank you for the previous answer about my Elantra 2002. Recently I computed the fuel consumption of the car. Unfortunately the result was not very good. I got 21.5 MPG. I know the "official" fuel consumption (33 MPG-Highway, 27-City) is a little optimistic but still what I obtain is rather bad.
Do you have any idea what could be the problem?
As additional information the catalytic converter was changed at a Hyundai service shop (two months ago).

Thank you.


Answer
Anything that could cause a lack of power, excessive engine rpm, or additional drag on the vehicle could affect fuel economy.  Additionally, driving conditions and habits can drastically affect economy.

Let's start with the converter.  What was it replaced, and if you know, which one was replaced?

Here'd be my approach to checking for causes of poor fuel economy:
1.  Check engine (ECM) and transmission (TCM) control modules for trouble codes.  If any are present, repair those items.
2.  Check all ECM and TCM data values and verify they appear normal.  It'll probably take someone with a professional scan tool to do this.
3.  Verify that the engine seems to be running normally and provide the proper amount of power.
4.  Verify the transmission shifts into all gears.
5.  Verify the torque converter clutch engages.  If you don't know how to do this, that'll probably require a professional also.
6.  Check tire pressures.  Higher pressures will increase fuel economy, but too high will cause premature tire wear in the center of the tread.
7.  Check the alignment and suspension.  If the toe in/out is too great, it'll cause the tires to drag across the pavement, resulting in increased rolling resistance.