A "fuel efficient" (gas) car needs to run the mixture at 16 parts air to 1 part gasoline. Any less air than that and there's unburned fuel in the exhaust.
There can be other, non-chemical reasons for inefficiencies though. The spark needs to be hot enough to generate the needed wave front. Some engines, like airplane engines, use 2 spark plugs. While mostly a safety/redundancy thing in airplanes, it also makes for a more complete "burn".
The timing of the spark and the condition of the valves also influence efficiency (not chemically however). Additionally the droplets of gasoline are more efficiently burned if they're very small (atomized); fuel injectors are better than the older carburetors for atomizing and mixing the fuel/air.
More significant to chemistry is that modern cars use a little computer to figure out what the mixture should be. They do this by reading an "O2 Sensor" which measures oxygen in the exhaust gas. See link below.
BTW, the above assumes that you want to use a standard, american type car powered by gasoline. You might write a better paper by challenging the gasoline assumption. Is your goal "gasoline efficient" or "money-efficient" or "eco efficient'?
