That mpg calculator is still a mystery to me. I've reset both trip odometers and both mileage screens immediately after filling up, and before I've even put the thing in gear and started rolling, it shows some impossible economy, like 18.4 mpg. No. It should be zero thus far. I've burned gasoline and gone nowhere. Zero miles divided by any amount of gas always yields zero mpg. So it's obviously basing it on some kind of history.
I've given up trying to figure out if it's calculated over some particular distance or if it's time or what. I went on a long trip last year (or was it 2 years ago?) and reset both trip odometers. Then each time I refueled I reset only one. I figured that at some magic moment, the two different fuel economies would align and stay matched. Then I would know for how many miles does it keep the running total. It never worked. Several times the numbers matched, but within a couple miles the two would diverge.
Which is all a long winded way of saying, don't trust it completely. It'll take you a few tanks of gas to really learn what kind of fuel economy you'll get. If you get stuck idling at lights a lot on your commute, single digit mpg is normal. If you drive sedately on highways and use the cruise, 21 or better is easy.
I see alot of folks getting 13 to 15. Love the truck so far!
