I believe the Active ECO Mode, which actually does something in the calibration of the transmission, throttle, and A/C, is unique to the 2.0T models. I believe the 2.4L Sonatas only have an "ECO gage", which essentially tells you when you're driving economically and when you're not.
Here is what I posted on another forum regarding the ECO mode button in our cars.
From a recovering automotive powertrain engineer...
The "ECO" buttons on most cars do essentially two things:
1. Change the transmission shift points to better optimize fuel economy at the expense of drivability. In other words, the transmission will usually shift sooner to keep RPM's down.
2. For cars with electronic throttle control (such as the Sonata), change the calibration of the gas pedal. Generally, in ECO mode, you have to push the gas pedal down farther to open the throttle the same amount. This encourages drivers to leave the throttle more closed and keep out of power levels that require fuel enrichment. However, even with the ECO button on, the throttle will still open all the way if you floor it. Here's a neat trick I noticed in my 2.0T - I had the ECO button on, and I held the gas pedal at a position that kept the car at a steady 70mph. I then turned off the ECO button. The car immediately started to accelerate, because the gas pedal calibration changed and opened the throttle wider (even though I hadn't moved the gas pedal).
Some manufacturers will also play around a bit with the air conditioning settings to squeeze out a few more fractions of MPG at the expense of A/C performance. Not sure if the Sonata does this or not. Depending on the manufacturer, there may be other minor engine calibration tweaks as well. However, Items 1 and 2 above are the big ones.
Basically, for stop-and-go city driving, the ECO button will make some difference in MPG for most drivers. If you are just rolling down the highway with the cruise control on, it won't make any difference in MPG.
Hope this helps...
What I would also say is that it's likely that ECO Mode results in less aggressive settings for the wastegate on our turbocharged cars. In other words, the ECU might hold the wastegate open more to prevent spooling of the turbo at low throttle positions to save fuel. Not sure about this, but l'm guessing it does.
Last edited by GN_Mike on 5/16/2011, 2:19 pm; edited 1 time in total (Reason for editing : added signature)