Gas mileage varies greatly on my 2014 TEP. I trust my vehicle computer, I used to check it manually but it has always been accurate so I don't break out the calculator anymore.
Everyday normal driving for me is a mix, but slanted towards the highway miles side. In that situation I get right around 20mpg, a little less if I have a heavy foot during that tank. I also usually get my gas from the same place, Costco.
On strictly highway driving I am all over the place, and I get very varied mileage depending on where I get gas. For instance, last weekend we were in Nags Head NC, then drove to the Hatteras lighthouse. On the way back I stopped at a BP station in Hatteras for gas. I filled up from half a tank with regular gas. Headed back down the road towards Virginia where we live. Now, it is a 55mph speed limit, but after driving 50 miles I looked at the trip computer and it read 27.3mpg. By the time we got home it had gone down to 24mpg, and yesterday it was at 22mpg and I have a quarter of a tank left.
This is not the first time I have gotten much better mileage with some gas from a station I don't normally buy from.
When the truck was a few months old we drove from VA to TX. After driving 4 hours from my house we stopped at a BP station in Mebane NC. I filled up with mid grade that had no ethanol. From there to outside of Atlanta I got 23.9 mpg. I could not believe it, so I double checked with a calculator, but it was correct. I then filled up at another BP station, with regular this time. This next tank I got a dismal 16.9mpg.