Modern engines run at 14.7 almost always, and they can hold it to within about 2% even on the most aggressive throttle movements and within about 0.03% on average. But that ratio is for emissions, and it really only gives the lowest emissions because it is balancing the chemistry (reduction of NOx, oxidation of CO and HC) happening in the catalyst. Timing also is tuned and not limited to vacuum advance, exhaust gas and valve and piston temperatures are measured during calibration and timing is set so everything stays below the burn-up temperatures, usually for lowest fuel consumption while avoiding knock. I had a 305 Chevy Caprice that would run at around 18:1 on the highway, but it also added around 5 or 6 degrees of timing when it switched to this “lean on cruise” mode. I got just under 30 US mpg with it.
