The rapid growth in gas prices over the past several years has finally stimulated Americans to take a serious interest in fuel saving measures. One positive side effect is that increased gas prices have reduced the amount of driving such that automobile related deaths are down by 33%. But for those still on the road, drivers are unfortunately sinking money into useless gadgets like fuel atomizers, air intake "swirlers" and fuel additives because they have been led to believe miles per gallon increases are possible upwards of 20%. These folks are usually duped into believing these simple technologies have been suppressed by a global automobile industrial conspiracy aimed at maintaining a dependence on fossil fuels. Idiots.
OBD II stands for On Board Diagnostics Version 2 and serves to monitor the components of an internal combustion engine that have direct influence on your emissions. Within your exhaust there is a sensor called the O2 sensor which measures the presence of oxygen in the exhaust to inform the engine computer whether the fuel is burning rich or lean. A "closed loop" exists between the engine computer and the O2 sensors because the amount of fuel delivered by the fuel injectors is dynamically controlled based on the detected level of fuel burn in the exhaust. Basically, your O2 sensor spits out a voltage between 0V and 1V and a value of 0.45V (standard on most sensors) indicates the stoichiometric balance of the air/fuel ratio for optimal burn.
Within the engine computer, there are trim variables that represent long and short term percentage changes in the injector timing designed to compensate for changes in the system as parts wear down, etc. For example, it is safe to assume that your spark plugs aren’t sparking as effectively after 29K miles and therefore a cylinder’s burn is no longer as complete as it used to be. This is detected as a change in your emissions (fuel mixture appears rich) by the O2 sensor. Here, the trim variables start tweaking down the injector pulse width slightly until the O2 sensor begins to report .45V again. Thus, your engine is compensating to adaptive conditions. Replacing the plugs now shows up to the O2 sensor as a lean burn so the trim variable begins to increase the pulse width until the sensor again reports .45V again. When those trim variables exceed some threshold like +/-20% … a CEL/MIL lights up.
The point of all this technical jargon is that the installation of fuel savings gadgets will have an effect of fuel efficiency in the short run. However, just as a change in fuel burn from worn spark plugs was detected by the O2 sensors, so is a change in fuel burn from improvement devices. Within the span of 500-1000 miles, the engine computer’s long and short term trim variables will recalibrate and adjust the fuel injectors such that emissions are restored. Often times this means fuel consumption goes right back where it was and sometimes results in even worse fuel economy.
This is somewhat of a reminder that the OBD II system was not designed with increased fuel efficiency as a goal – it was designed to appease the atmospheric environmentalists of the 80’s and 90’s. We could always get better fuel efficiency by saying to hell with the atmosphere …. it just depends on which environmental/political movement is strongest at the time.
There are only a few real courses of action one can take to improve the fuel efficiency on a given vehicle. All of these measures are thoroughly documented in Improve MPG: The Factors Affecting Fuel Efficiency. The first is to change driving style! That means slowing down to a speed whereupon the vehicle’s torque output in an overdrive gear is providing just enough power to force the vehicle through a given speed’s air resistance. A national initiative for renewing the 55mph highway standard is once again on the table. For many cars, this will be a very effective measure but there are cars that perform better at slightly higher speeds. Aside from altering driving habits, motorists can replace or modify their vehicle’s engine computer. This requires either having a custom programming uploaded into the computer or a "piggyback" computer altering the stock output signals. Of course, this is only effective if the modifications actually correspond to modifications actually installed on the vehicle. Such a programming can be achieved by professionals using a dynometer.
But if you do insist on wasting your money on the truly useless modifications, I will sell you a rock and some duct tape for $50 plus shipping and handling. I will include instructions on how to install this fuel saving device onto the backside of your throttle pedal to limit your ability to floor it and thereby save you significant amounts of gas. As an added bonus, I will even personally guarantee your rock will be artistically unique in shape but is also environmentally sound – made from 100% natural material.