What Fuel Mixtures do Modern cars run?

All modern gasoline engines run at the chemically-correct fuel-air mixture (called stoichiometric, 14.7:1 air/fuel) almost all of the time. Chemically-correct means that there is just enough Oxygen to burn all of the fuel. The reason for this is:
The emissions-control system, consisting of the catalytic converter and Oxygen sensor(s), only works at this ratio. The
exceptions to running stoichiometric are:
1. Cold-start; engines have to run rich for a few seconds during and after a cold-start-up. Prior to modern fuel injection, this rich warm-up mode could last for minutes. The best Direct-injected engines today revert to Stoiciometric after just a few seconds.
2. Full power; An engine can make more power by running rich at any speed, but at moderate-to-high engine speed, running rich can help keep the exhaust temperature down to avoid damaging the exhaust valves, exhaust manifold, and catalytic converter.
3. Lean; Any engine will run more-efficiently if running a lean mixture. Lean means excess air. There have been lean-burn gasoline engines in production, and Diesels have to run lean; but lean-burn is a huge challenge for emissions because a lean burn engine still emits a little bit of Hydrocarbon and NOx and a normal catalytic converter just doesn’t work when fed lean-burn exhaust. To make a lean-burn engine legally-clean requires an extremely expensive exhaust after-treatment system. So modern gasoline engines do not run lean.

Mazda’s new SkyActive-X engine is expected to be the first new lean-burn engine in decades, but you will see that it will have a big expensive and complex exhaust aftertreatment under the car (if it actually makes it to the US market).