Modern cars circa '96 do run at stiochiometric as soon as possible...
Originally Posted by dreadknought
and as much as possible because this represents the ideal mixture of air/fuel that produces the exhaust gases best processed by the 3-way converters.
Under non-open loop operation (cold start, hard acceleration, closed throttle coast down) the controller can go open loop and then can feed teh engine nearly any air/fuel mixture it wants.
All fuel consumption calculations are going to be imprecise unless one knows the weight the gas being consumed to a precise value. Gas weight varies due to differences in blends that arise from octane differences, or gas targeted for a season or a region that has some botique blend requirement and temperature to name a few factors that I can think of. There could be more.
But fuel consumption based on air consumption is probably good enough for all but labs needs.