I have a Hemi with the 3.92. That is about the same as what I get at that speed. I.e. struggle to get 19 if I'm cruising at 70. Struggle and generally fail.
But, I pretty much never cruise at 70, either. If I'm on the road, that means Interstate and cruise set at 80. Mileage really drops off the more you exceed 70. I get about 17 when cruise is set at 80. That is hand calculated using the Fuelly app at every fill-up. The onboard lie-o-meter ALWAYS says I got better than I really did. My last two highway-only runs were:
Actual 17.4MPG, computer said 18.5.
Actual 17.2MPG, computer said 18.2.