Well, it's been 24 days with my truck and, last night I filled up the tank for the 2nd time. The MPG average in the instrument cluster had been telling me 19.3 average, which I thought was a bit on the high side, even for a single-cab-long-bed.
My driving has been mostly side roads of around 40-50mph, a stop sign or red light every couple to few miles or so...and a couple trips of about 30 miles on the highway at 65 mph. My tank averaged exactly 18.03 miles per gallon...I fill this truck like I would my F150, on medium speed and stop after the first click...no squeezing off one more click. It took 22.180 gallons to go exactly 400 miles...18.03mpg. I'll take it!!!
Even with the ~0.30 / gallon diesel price penalty over 87 octane unleaded, I still netted a 14% gain in economy / savings in fuel cost per mile over my 2006 F150 5.4L V8. Outstanding. With only 3019 miles on the odo, it should only get better as it breaks in.
I use an Android app on my phone called "FillUp". It was free and extremely simple to use. It only has the bare essential fields of data, let's you track multiple vehicles, let's you export a CSV file for tracking on a spreadsheet, backing up data, etc.
As an interesting side note, my 2006 F150 Lariat had an average MPG indicator that used a rolling "last 500 miles" calculation in the computer. It was NEVER more than 0.1 MPG off the hand-calculated average. I wonder why Ram's calculation is so sketchy. Also, my Ram's "Trip 2" mileage log has never been reset, it matches the odometer for miles. It too reads about 19.1 MPG average.
Looking forward to many years and many miles with this truck - love driving it.