If you've shopped for a new car in the past year or two, you may have noticed a new feature that's creeping into all sorts of models: The "eco button." Manufacturers are installing the little, illuminated squares of plastic in everything from hybrids to sporty coupes, high-horsepower sedans to V8-powered SUVs.

Ostensibly this new feature allows you to transform your vehicle into a green machine, selecting a driving mode that puts your car's abilities on a short leash -- but only at your choosing. Depending on the vehicle, in green mode the throttle won't be as responsive or the transmission will shift into a higher gear at lower rpms, or the electric systems will be tuned to reduce their energy draw.

Here are a few examples of how these eco modes affect the driving experience:

Chevrolet: The four-cylinder Equinox with a six-speed automatic transmission comes with an eco button that, when pressed, locks the transmission's torque converter at lower speeds and tells the transmission to shift to a higher gear earlier. It has no affect on throttle response.

Dodge: The 2011 Grand Caravan in eco mode will smooth out throttle response and upshift sooner in order to save fuel.

Honda: The CR-Z and Insight, both hybrids, have eco buttons and the feature will be present in the Fit EV to increase battery range. In economy mode for the CR–Z and Insight, the electric motor assist prioritizes fuel efficiency and the air conditioning system will reduce its overall load on the engine. The drive-by-wire throttle is optimized for smoother acceleration and maintains the lowest possible engine rpm, power and torque decline by four percent (except at wide-open throttle, which still gives full responsiveness), on the CVT-equipped models the transmission ratios are optimized to be higher relative to engine rpm, and when using cruise control the throttle employs a smaller opening angle whenever possible.

Hyundai: The Korean firm's eco offerings are shaded blue rather than green, so the Sonata has an Active Eco button and so will the coming Veloster. It modifies the transmission shifting schedule.

Infiniti: The 2011 Infiniti M – with a 5.6–liter, 420-horsepower V8 – comes standard with an eco button, but its operation can be enhanced if a buyer chooses the optional Technology Package. The standard setting changes throttle positioning and shift points. Get the upgrade, however, and you'll get the "Eco Pedal" (which can be turned on or off) that "provides feedback to encourage the driver to optimize fuel efficiency." Translation: The accelerator pedal pushes back a bit and vibrates underfoot when you're giving it the lead boot. "The feedback is slight," Infiniti says, "and can be easily overridden by the driver depressing further on the pedal."

Nissan: The Juke crossover has an Eco button, which adjusts throttle responsiveness and transmission mapping for the CVT. And in what seems like a case of miserliness heaped atop frugality, the electric Nissan Leaf also has an eco button. Pressing the Leaf's button has the same effect as the Juke, but instead of changing the transmission mapping – since it doesn't have a traditional transmission – it provides more aggressive regenerative braking, as well as reducing the use of the air conditioning compressor.

Toyota and Lexus: The Toyota Prius and Highlander Hybrid, and Lexus HS250, RX 450h, LS 600 hL and the coming CT 200h have switchable eco modes that tweak accelerator response and optimize the climate control. The Camry Hybrid also has a dedicated green mode, but it doesn't affect the throttle, it only adjusts HVAC system to improve fuel efficiency.

When it comes to quantifying the results of driving in these economy mode, manufacturers usually estimate a five- to ten-percent improvement, though Dodge said that the econ mode in the Grand Caravan is good for about one more mile per gallon. Do the math and a vehicle rated at 25 combined mpg will then get anywhere from 26.25 to 27.5 mpg. That's about the same improvement most people would see if they kept their car in a proper state of tune with the tires inflated -- or if they just drove a bit less aggressively.

Of course, you mileage will vary, as is the case with all measures of fuel efficiency, including the official ones done by the EPA. Which brings up another interesting point, which is that all the vehicles hauled off to the EPA's National Vehicles and Fuel Emissions Laboratory in Ann Arbor, Michigan, for testing will be revved on the dyno only in their normal operating mode. Even if a car is equipped with an button that adjusts the car's responses to increase fuel economy, the EPA never presses it to ascertain how mpg is affected.

There are plenty of reasons for this, prime among them that fuel economy and emissions testing protocols are so interrelated between the EPA, Department of Transportation and Congress that the proverbial butterfly shifting its wings in EPA headquarters at Federal Triangle could translate into chaos in the Capitol. But another reason is even simpler: It isn't clear who really uses the eco button, and how often, and why. Even the manufacturers aren't sure.

When we asked, only Honda provided us with an estimate, saying "our internal customer research shows that in the CR-Z, 50 percent of drivers use the ECO button (compared to Sport and Normal) as their primary driving mode." That's a healthy number, and while we have no proof of it, we'd tend to think that the usage stats for other cars wouldn't be that high. We suspect that buyers of the CR-Z, a small two–seat hybrid, are more inclined to use their economy driving mode than those who take home an eight–cylinder Infiniti M.

The other big question is why these buttons exist at all? Is it because car buyers really want them or has marketing taken over the movement? It's probably a dead heat. Buyers really are asking for features that will let them spend less at the pump, and eco buttons can save gas. If oil prices continue their climb, buyers' polite requests will surely turn into forceful demands. Nevertheless, drivers don't want to sacrifice speed or comfort or ride, at least not all the time. Hence the eco button, so you can go further, if a little bit slower. It might be marketing, but since it leaves things entirely up to you it is also, in the words of Kyle Bazemore at Infiniti, "the luxury of choice."