Fuel mileage is a very tricky thing. There's only one way to accurately measure it, and even that way has its shortcomings. Drive a certain amount of miles, make sure the speedometer is accurate, and then fill it up with the same amount of clicks after it's full.
I always always always check my fuel mileage, I've been doing it since I first started driving, because my father always did it. And I drive across the country two or three times a year, and there's times when I do the exact same fill up routine, and my mileage will drop 10 or 20%, or be 10 or 20% higher. And I have no explanation. From one fill up to the next I'll go from for example 15 MPG, to 19. The next fill up will be back down to 15. Makes no sense whatsoever, but one thing I have noticed, is when I just put in a gallon or two or three, the mileage can vary drastically. And the reason I only put in a gallon or two, it's maybe because the fuel price in the next state is a lot higher, or I want to have it full when I start off driving the next morning.
So I never take it seriously unless it's a half a tank or more, usually the best is when the tank is empty. The margin of error seems to be less.
My 2008 G5 has an electronic fuel mileage counter, and by the way NONE of these actually measure the fuel going through your engine, they're using an algorithm based on computer simulations in a lab. And I check my fuel mileage every fill-up, and I drive the car a lot, and sometimes it's within a very small error, and sometimes it's off by 5%. So no explanation for that either except that like I mentioned, it's not the actual fuel passing through the fuel line it's measuring, it's just checking all the parameters and basing the dispay you see on a computer model.
Back on topic, my '84 6.2 diesel Suburban with a 700 overdrive and no lockup gets around 16 17 at 60 miles per hour. It's a four-wheel drive 3/4 ton, and my 6.5 turbo diesel pickup truck used to get around 15.5 or 16 with lock up. So less heavy pickup with a turbo diesel would get slightly lower fuel mileage. And my '95 6.5 turbo diesel 3/4 ton four-wheel-drive Suburban got the exact same mileage as my '84 6.2, even though the '95 Suburban had lockup, and my '84 lockup wasn't working at the time. Now I have a rebuilt transmission with a hydraulic lock up in the '84 6.2