Would running lean save fuel?
 

I know that an air fuel ratio of 14.7:1 is in stoichiomitry, obviously running rich burns more fuel, and running lean reduces performance. But what if a car ran at 15.5 or 16:1 air to fuel ratio at all times, except when cold, under heavy load or at full throttle? I would think it would get better mileage, at the cost of worse throttle response and higher temps. Why is the air to fuel ratio so rich while cruising? Could the engine not do the same amount of work with a leaner air to fuel ratio, or would fuel consumption increase? Thanks, this always confused me.

Law and emissions

This is where your EGR helps...lets you run less air and fuel without having to change your AFR.

I tuned the carburetor on my gasoline powered suburban with the aid of my AEM wide band O2 meter for lean burn and it made about a 20% improvement over a properly tuned carb.
It works.

You do not want lean burn at high load, I burned all the exhaust valves on a fuel injected engine because I didn't know how to make it run rich at high load.

Makes you wonder if having your car make a little more NOx but reducing fuel use 20% might reduce overall emissions since your taking a chunk out of the 300 grams of NOx per gallon the refinery makes?

That matches my thought process. Not sure how much NOx a car in lean burn mode would produce. But 20% less fuel use should make up for that in my eyes especially if what your saying about NOx the refinery makes is accurate.

Where I live we do not have a NOx problem.
It's flat as far as you can see and one of the windest places in the united states.

Unfortunately EPA only cares about vehicle tail pipe.
I had no idea the refinery made 300gm of NOx per gallon.
Heck it's almost like the government is intentionally making the problem worse.

I have been around oil refineries and they do smell like NOx, but I didn't even think it was possible to release that much.

That sounds extremely high. Do You have a source for this?

If you do this on a 25-year-old car in USA then I'm pretty sure you're in the clear when it comes to emissions regulations.

Anyone who thinks the government produces regulations to help the average person is crazy. Maybe 100 years ago. The regulations are written by lobbies and special interest groups to carve out an advantage for themselves. Who benefits from using more gas and adding expensive parts to cars? In the name of reducing NOx which supposedly helps the common man. Must be nice every time the common man gets "helped" a CEO makes another billion.