Mileage on 99 compared to a 13

[Felber61]

New to the forum but we have a 1999 XLT 4x4 that's been the most reliable rig we've ever owned (after one ejected spark plug) After a few other brands my wife opted to go back to an Expedition XLT 4x4. This one is a 2013. The question I have regards mpg. Our 99 got 15mpg on the highway religiously and this 13 shows 8.5-10mpg on the dummy calculator on trips of 140 miles highway and 100 mile highway. I don't pay attention in town because everything gets bad mileage in town. Is the dummy calculator that far off or am I doomed to keep pumping gas in this thing by the day? We've only had it for about a month so I'm unsure of the gearing. I literally just signed papers. She had the deal all done when I got back in town from work. Everything I found on the forum says it should be closer to 15-17 highway. I'm just worried I got a lemon. other than the ranch hand grill guard its stock with 26k.