Consumer Reports bought and tested the ECO version of three small sedans to figure out how much fuel economy improvement they showed compared to the regular versions. You can see and overview of the tests in the video above, and the exact results for each model in the table below. The results vary pretty widely, with the Ford Focus SFE being the best value since it has a smaller price differential with the regular Focus than the others and provides a bigger improvement in fuel efficiency. The Civic HF also provides a decent (10%) improvement in fuel economy, but the Cruze eco disappoints with only a 1 MPG improvement in CR's tests.
Here's Where I Go Off on a Little Rant...It has always bugged me a bit that anything that makes a car greener or more fuel efficient gets judged purely on the criteria of "will it pay for itself" by the popular auto press. Most options on a car are pure costs. If you get a bigger engine, a luxury trim, a sunroof, fancy wheels, etc.. All of that won't give you back any money, and the bigger engine will actually keep costing you more over the whole life of the vehicle. So you'd think that fuel economy improvements could, at least partly, be judged on the basis of "hey, you're burning less gas, isn't that cool?" rather than "will it pay back for itself?".
But even that payback question is usually looked at the wrong way, in my opinion. For example, the Ford Focus SFE costs $495 more than the regular version and, according to CR, saves on average $145 a year in gasoline (could be more if prices keep going up). Now at this point most auto journalists kind of go, "495 vs 145, not really worth it".
But that's not a sane way to look at return on an investment. Nobody invests money expecting to get 100% back in interest within a year. Right now if you put your money in a "high interest" savings account, you'll be lucky to get 2-3 percent every year. So $145 back per year on a $495 investment, which is a return of 29.3%, is pretty damn good. After 3 years you'll have paid for it, and then for the rest of the life of the vehicle, it'll be pure profit, on top of polluting less. Seems like a pretty good deal to me.
So there can be good reasons why the eco version of a car isn't worth it, but to find that out, it's best to figure out what criteria matter most to you. And if you decide that how fast it pays for itself matters, look at it as a yearly percentage of return on invested capital to get an idea of how it compares to other investments.