Interesting observation. Not trying to discount it, but there are also other effects that could be at work here: break-in effects, summer gas formulations, maybe your area changes from a 10% ethanol in the winter to none in the summer. Warmer weather also means engine starts warmer and warms up very slightly quicker on start and you might be using the defroster less as well. Longer daylight means less use of the lights. And so on. I've tracked mileage on a dozen vehicles we've owned for over 20 years and, back when we lived in a non-tropical climate, every vehicle had a several MPG swing summer to winter. That data set also includes 8 Honda/Acuras from new all of which had a noticeable and more or less step increase in MPG somewhere in the 10-20k miles range I attribute to break-in.

If you wait to experiment with going back to Econ until the winter, you will have all the winter-summer issues at play that will create "noise" in your data. Econ may be adding 1 MPG and winter conditions you haven't controlled for may be costing 2 MPG. Net -1. Econ mode bad? Not so fast.

If the Econ effect were in the 10-20% range, it might be easier to spot. In the 0-5% range, there are just a lot of other things that can be going on that get in the way of drawing good conclusions.

 

My wife too wanted the LX model for the extra mpg over the EX. None of the dealers in my area had a CVT LX so I got the EX. My wife has that eco button turned on all the time. She said with combined driving, it is at least 4-5mpg better than the 09 GE she use to drive.

 

Being Canadian, My number s are a little different, I bought the car with 6,000 km on it, and have had the econ ON since I bought the car.

the I have had the car, for 6 weeks, I have not put gas in the car yet.
my trip calculator states, 8.4L/100 km? (I think 30-33 mpg)

The last trip (previous owner, was down to 5.7 (42 mpg)

So it was a fleet, so I would assume they drove it hard, I drive like I am afraid to burn fuel.

maybe they are on highway more? or longer trips?

but I drive with a soft foot, and eco mode, and I get 10 mpg less than the other guy.

 

This is my first car with a real-time fuel economy readout. I've noticed that it has changed my driving style. Gradually, my numbers on the city streets and the highway are improving.