I've completely read through my owners manual and done some Googling with no worthwhile results. So far I like the feel of ECO mode better than non-ECO, and I see no mpg difference either way. I am generally very light on the pedal anyway, and always seem to exceed the EPA ratings on any car I drive when tracking mpgs. I'm sure everyone will say go with what I like, but ultimately I want what is best for the car. But I have no idea what ECO mode actually changes. Does it retard timing, change shift points, or cause any hidden issues? Something else entirely? I love this car, but truly hate how vague Hyundai is about everything on this car. Why does it matter? Well, using my Impala as an example, lets say it has an ECO button (it doesn't). If the ECO button activates AFM (cylinder deactivation) when on, some may like it, some may not. But in my case, AFM caused it to burn a quart of oil every 700 miles. Not good for the car or my wallet. Obviously YMMV from mine, but at least some people know/complain about the hidden problem. So I fixed the problem by tune-disabling AFM. Life is wonderful and I burn no oil at all anymore. If ECO causes any issues at all (even petty ones that long-term may be a nuisance), I'd rather just leave it off. Does ECO mode have any strenghts or weaknesses? Is ECO better in one driving style over another (city/highway, <40mph/>40mph) Hopefully someone can chime in with details. Thanks in advance!