> New vehicle will burn fairly completely (if you include the cat as part of the process), leaving a miniscule amount of hydrocarbons (incompletely burned fuel)

> New vehicle will burn fairly completely (if you include the cat as part of the process), leaving a miniscule amount of hydrocarbons (incompletely burned fuel) and a small amount of CO So will old ones, if you run them on the right fuel. My 25-year-old Range Rover has a 4.6 litre engine adapted to run on propane which is - under the right circumstances - a *negative* emission vehicle. It can have measurably less CO and HC in the exhaust than in the intake air (city air heavily contaminated with CO and HC for example from bus engines).