By far the #1 factor: Speed.
As stated by others, drag is a square function. So 2x the speed = 4x the drag. If you want good fuel economy and don't care about making enemies on the interstate, you can follow a semi. You don't have to tailgate to see gains. Even following at a reasonable distance will help.
When cruising down the road, ECON helps your fuel economy slightly, but maybe not in the way you may think. The most obvious thing that ECON does is re-map the throttle pedal, giving a much slower start. When cruising down the road (especially with cruise control on), this does virtually nothing for fuel economy. But what also happens is that ECON mode slightly "loosens up" your cruise setting, allowing your Ridgeline some leeway. So when going up a hill, your speed may decrease a bit more in ECON mode. This immediately gets back to the first point - speed. Because ECON allows slightly slower speed going up hills, you'll get slightly better fuel economy because of the reduced drag. You certainly don't need ECON mode to achieve the same results. If you just drive 1 or 2 MPH slower, you'll be able to beat the ECON gains. It's just that ECON does this for you automatically. The other thing ECON does is to alter your air conditioning. ECON reduces the cabin fan speed which causes less heat transfer across the evaporator which causes longer delays between the compressor running. I
think I read somewhere that ECON also allows wider evaporator temperature swings which also causes less frequent compressor cycling. The point is that ECON produces less robust air conditioning for a slight economy gain.
Outside air temperature plays a role, too. I commute every work day over 40 miles each way, and we take the Ridgeline on road trips because it is so comfortable. So I put well over 2,000 miles per month on my Ridgeline. I've been keeping close track of my fuel economy since last June. This graph is my hand calculated economy. On average since last June, my dash indicated economy is 0.7 MPG higher than my hand calculation. As you can see, outside air temperature plays a fairly significant role:
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Note that I have my Ridgeline set up to automatically disable Auto Idle Stop using a time delay relay to automatically turn my power inverter on, and I have an S-VCM Controller to defeat Variable Cylinder Management. I drive 100% with ECON turned OFF. I carry around lots of tools, so my vehicle weight is probably higher than most.
I don't intend to be an ECO or Auto Idle Stop apologist, I just like hard data on what actually works to maximize MPG and minimize fuel cost.
