I only have a little more than 2,000 miles on my F3-S but I think I have figured out how the ECO mode works.
If you ride at a constant speed (cruise control) you will not see much of a difference between normal mode and ECO mode. This makes sense because ECO mode does two things that do not apply to steady-state riding. ECO mode encourages you to short-shift and the throttle "curve" is non-linear. (Both throttle curves are non-linear, but the ECO curve is much more "restrictive".)
I have run some tests to determine what causes my Spyder to use more fuel at times. The two biggest causes are accelerating briskly and travelling at higher speeds.
Most of my riding is with the cruise control on. (I use the cruise control even at speeds around 30-35 MPH.) I have had tanks at 45 MPG and tanks at 35 MPG. The difference is the speed I travel at.
This week I did two 210 mile trips. The trips were mostly steady-state cruising, anywhere from 45 MPH to 70 MPH. Both trips were the exact same loop route, same temperature, same humidity and same time of day. I use 93 octane ethanol-free fuel. On trip one I used sixth gear for cruising any time I was at 45 MPH or more. On trip two I used fifth gear any time I was at 45 MPH or more. I got 39.4 MPG on the first trip and 36.6 MPG on the second trip. So just lowering the engine RPM at cruise speeds is worth 3 MPG.
I live where it is flat with some occasional rolling hills, so I can usually cruise in sixth easily at 45 MPH or higher, which is the gear that ECO mode usually suggests. In rolling hills I use fifth until 55 MPH. The engine is nowhere close to lugging when cruising in sixth at 45 MPH at 2600 RPM, as long as the road is level and I am riding solo. When I am out in the middle of nowhere I take my time so I can enjoy the ride. Sometimes I will not see another vehicle for many miles.
At first I thought the ECO mode was just some programmed shift points but I now believe that it uses engine load to help determine when to shift. I get different suggestions when I have a passenger or I am on a hill or if the engine is not warmed up.
Most of the time ECO mode does not make my riding less fun. My usage pattern is long steady-state rides on long rural roads. I rarely ride for less than an hour.
I have owned 24 new bikes and thus far the F3-S has the best fuelling that I have ever experienced.
Mentally I play a game where I try to get the best mileage possible. Simple things like rolling to a stop or accelerating a bit slower or timing a red light to prevent having to use the brakes yields huge mileage improvements.
There are times I want to go much quicker and this is reflected in much lower mileage figures. I feel less of a need to ride quickly on my Spyder because is it significantly slower (acceleration and top speed) than most of the motorcycles I have had. For me the Spyder is a leisurely cruising/touring machine.
I log all of my mileage here:
I have submitted the technical (gearing) data for the F3 to gearingcommander.com so that you can compare engine speeds in different gears and stuff like that.
I am really impressed with the ACE 1330 engine so far. It is very smooth, has plenty of torque and has decent fuel economy.
YMMV