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I know how to adjust the air/fuel ratio for older cars with carburetors, but how would I go about adjusting it on a 2001 civic?

You don't adjust air/fuel ratio. It is controlled by computer, and the oxygen sensor(s) are used to determine whether the mixture is too rich or too lean. Mixture, idle speed, and ignition timing are now all automated and are not adjustable. If the car has a too rich or too lean condition, check the oxygen sensor (or sensors - some cars have two).

You don't adjust air/fuel ratio. It is controlled by computer, and the oxygen sensor(s) are used to determine whether the mixture is too rich or too lean. Mixture, idle speed, and ignition timing are now all automated and are not adjustable. If the car has a too rich or too lean condition, check the oxygen sensor (or sensors - some cars have two).

While I agree, for the most part, there is a method of changing air/fuel ratio. In racing, the engine ECM is frequently replaced or it's program is modified to produce more horsepower. Typically a highly modified engine will require adjustment of the fuel/air ratio. There is nothing to say that the same methods could not be used for hypermilers, but this approach has lots of built in risk. There is a saying that "lean is mean," meaning that many engines make more power when the mixture is leaner. But, along with this approach comes lots of risk. An overly lean engine is prone to detonation and to holed or burned pistons. This frequently happens when a racing tuner gets just a bit too greedy.

The manufacturer has set your air/fuel ratio to a compromise value which is viewed as engine safe over a wide set of operating conditions. For an unmodified street car, it is a bit of a stretch to say that an individual tuner can find a better compromise.

There are approaches which one MIGHT safely use to lean the mixture, but they are expensive and would probably require a chassis dyno tuning expert. In addition one would need to change the computer and buy a broadband O2 sensor with dash display - to be safe with the new tune.

We're doing this with racing Miatas, but we have also burned some pistons;)

look into software that plugs into your car and your computer. Dont know whats available for the hondas, but I use efilive.com on my chevy (doesnt work on Hondas, sorry) for making adjustments to the ecm and tcm. Theres got to be something available for the honda.

Some ECM/ECU are reprogramable, some are not. But that is not the real issue. The real issue is being able to make intelligent and known changes. Just making changes on a hunch or just because some seller (i.e. chip shill) says it is "good" is an invitation to disaster. It is fairly easy to make a bit more power by leaning out the mixture or advancing the timing, or both, but unless you have a large budget, standby mechanics, and considerable understanding you run the risk of a blown motor. I just wouldn't go there without some expensive engine testing and some in car gauges. I've seen it go wrong, and it isn't pretty.

Some folks were selling gimmick "resistors" that were supposed to allow you to trick your ECU into leaning the mixture by making it think more air was coming in than actually was-sending wrong info from the mass air sensor.
I can't picture this actually working since the O2 sensors would keep telling the ECU that it was too lean.
Besides-as others point out- if it did work you will probably blow a hle in your piston-maybe during one of those 100 degree days you guys had last summer.
Not worth trying to "beat" the Honda engineers-tey know what they are doing.
Drop some weigh-pull the back seat or delete the spare-carry plug kit-pump-cross your fingers.
Charlie

what are you trying to do?