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ECM mileage is based on the number of transmission output shaft rotations, and is calculated along with the programmed rear end gear ratio, transmission gear ratios, and revolutions per mile for the programmed tire size. Actual mileage will vary according to tire wear... as your tires wear down, the actual mileage you travel will decrease.
Unless it's an all-out racing chip, which I doubt, I would think your driving habits would affect gas mileage much more than just changing the ECM chip.
The -E-ngine -C-ontrol -M-odule, or ECM recorded the vehicle mileage along with the odometer. The EMC mileage can be read by a scan tool at most dealerships or repair shops with the proper equipment. It is possible for the two mileages to differ slightly, but a large variance may indicate a faulty odometer, or possibly odometer tampering.
You should have your ECM changed so that you are not using too much gas. If you don't switch out your ECM your car will get worse and drink more gas.
Where did u get your new ECM? You probably need to have it " flashed" @ a gm service dept. Or a shop with tec2 scanner
Google Jet-Chip that is all I have found so far.
check pickup in the distributor
you may have changed the tire size which will give you incorrect mileage hence causing an erroneous gas mileage reading.
i had mine changed in Jan 2011 mileage 48000
check the pickup coil. that was the prob
check recommended mileage, DO NOT EXCEED
100K