do you want to reduce the CO2 emissions from your car?
then remove the cats. the pre cat CO2 reading are typically around 2~3 points lower than the post cat CO2 reading are.

Josh wrote:
The idea of a cat adding anything to the exhaust (adding Co2 as he states) is not correct. A converter can not add anything to the exhaust.

Josh wrote:
CO2 (Carbon Dioxide) isn't environmentally harmful, ...

Three-way catalytic converters

A three-way catalytic converter has three simultaneous tasks:

1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + 2xO2 → xCO2 + 2xH2O

A two-way catalytic converter has two simultaneous tasks:

1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
2. Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: CxH4x + 2xO2 → xCO2 + 2xH2O

Environmental impact

Catalytic converters have proven to be reliable devices and have been successful in reducing noxious tailpipe emissions. However, they may have some adverse environmental impacts in use:

* The requirement for a rich burn engine to run at the stoichiometric point means it uses more fuel than a "lean burn" engine running at a mixture of 20:1 or less. This increases the amount of fossil fuel consumed and the carbon dioxide emissions of the vehicle. However, NOx control on lean burn engines is problematic at best, and many lean burn engine manufacturers are considering rich burn variations.[citation needed]
* The manufacturing of catalytic converters requires palladium and/or platinum; a portion of the world supply of these precious metals is produced near the Russian city of Norilsk, with significant negative environmental effects.[5]