I have a bit over 10,000 miles on my '12 Limited. I have never reset the fuel economy calculator, or trip mileage. So I thought that the fuel economy it was showing me was all time since I acquired the vehicle. That showed 17.0. Most of my driving is local. I just got back from a 1,000 mile trip, most of which was on highways and Interstates. My economy now shows as 21.3. I guess I find it hard to believe that one 1,000 mile trip can improve all time economy by that much.
Was the 17 mpg before the trip really the average number since I first got the vehicle? Should one highway trip make that much of a difference?
As folks have mentioned, the fuel economy display is a running average with a "window" of x miles. So yes, a 1000 mile highway trip will get you to where you are more-or-less showing what your highway mileage was since the "window" is undoubtedly lower than 1000 miles in my experience. The accuracy can vary slightly from "real" hard-measured fuel economy, depending on how you drive. I have a light foot and my EVIC mileage has actually been pretty close to "real" based on my spot checks using a calculator and fuel recepts. I'm also showing 17 for my general rural driving right now; slightly higher than it was because with the milder weather, I'm running with the HVAC on manual with just outside air blowing in. Highway, I get about 20-21 manually calculated as well as via the EVIC.
2012 Grand Cherokee Overland Summit 5.7L V8 - Winter Chill with New Saddle Interior - Nitto NT421Q
I think your numbers are reasonably accurate.
I just did a road trip from Toronto Canada to Lawrence Indiana USA.
EVIC read 12 MPG at start of the trip and 23 MPG at the end. I stopped 3 times along the way.
Did the same trip in the wife's 2 door, 2 liter Hyundai had almost the same numbers
The MPG will creep up and I would think that a 1000 mile trip would be far enough to raise it to 21 mpg. I know that when on the hwy driving the speed limit or a little above and not passing a lot I will always get 24+ mpg (V6) and around town I get between 18 & 21. I think the GC gets pretty good mileage considering what you are driving. There are a lot of lesser vehicles out there that don't get all that much better mileage!