Question

Is it true that a catalytic converter reduces fuel efficiency? If so, by how many percents does it reduce?

Answer 1

It is true that a catalytic converter reduces fuel efficiency. A cold engine when first started may have fuel efficiency reduced by `50%` and up to `10%` more `CO_2` can be produced when idling.

Explanation:

The catalytic converter was added to production vehicles after the removal of lead from gasoline to produce the unleaded gas we use now. Catalytic converters work three ways to reduce harmful emissions in exhaust gases. They reduce oxides of nitrogen to oxygen and nitrogen. They oxidize `CO to CO_2`, and they oxidize hydrocarbons to `CO_2` and water.

But the catalytic converter must reach its operating temperature of `750^0` to `1000^0C` and it must remain inside that stoichiometric combustion range. That means cold starts, idling, and rapid acceleration will all result in lower fuel efficiency. The catalytic converter also restricts the exhaust gasses from flowing out of the engine which also contributes to inefficiency.

Information is here under Environmental impact:
https://en.wikipedia.org/wiki/Catalytic_converter

Fuel consumption is discussed here:
http://automotiverecycling.net/catalytic-pollution.shtml