If the water is not counted, one seventh of the exhaust, 14%, is carbon dioxide (CO2), the rest nitrogen. Modern cars have an oxygen sensor and computer system that, when hot, carefully measure a small excess of oxygen in the exhaust and meter the fuel so as to maintain close to exact stoichiometry. The extra hydrocarbon caused by the cold walls is burned on the catalyst by the small excess of oxygen.

When a vehicle is operating exactly at stoichiometry, the chemical equation stated previously can be used to calculate the pounds of air used to burn a pound of fuel. The result of this calculation is 14.7. Thus, when you fill your car up with 50 pounds (about eight gallons) of gasoline, this enables your car to process almost 750 pounds of air. Imagine how sluggish your car would be if it had to carry those 750 pounds of air. Electric cars have to carry around all the energy it takes for their propulsion. That is why electric cars have such a short range.

If the car's computer and oxygen sensor system are not working, then either there is too much air (fuel lean), or too little (fuel rich). Lean mixtures often cause misfiring, hesitation, "coughing," and thus poor fuel economy and poor performance. Rich mixtures allow cars to perform well except for loss of fuel economy and higher emissions. Vehicles that break are often programmed to go intentionally to rich mixtures. Suppose somehow 33% less air gets to the engine. The 1.5 (O2+4N2) becomes just (O2+4N2). The combustion equation then becomes the following:

All the carbon dioxide has gone and has been replaced by 20% carbon monoxide (CO). Most cars cannot run that rich, but notice how a relatively small deficit in air (or excess of fuel) leads to a 100% increase in CO. Even 10% CO is enough to kill someone breathing it in a few minutes.