[BigFreakinToad]

I guess I should have listened better in chemistry class, but I do seem much more water running out of the tailpipes of modern cars than I recall in the days of leaded gas.

[Yo-Yo]

I guess I should have listened better in chemistry class, but I do seem much more water running out of the tailpipes of modern cars than I recall in the days of leaded gas.

No doubt you do see more water dripping out of cars, because today's engines are more efficient and you are correct that catalytic converters do produce some water. But the vast majority of water vapor produced is from combustion.

The perfect chemical reaction is HC + O₂ = CO₂ + H₂O.

The real world imperfect combustion also has Nitrogen Oxides (NO_X), Carbon Monoxide (CO), and other byproducts which the catalytic converter helps to convert to Nitrogen (N₂), CO₂ and H₂O.