Love your thread Trailmaker. I should do this on my next road trip. I've been keeping a MPG mileage log since I first learned to drive back in the 70's. (Miles driven divided by gallons to top off = MPG.) I have done this religiously at every fuel stop. Pros for doing it manually: I feel it offers a truer and more accurate reading than the electronic gauge; when done at every stop, the driver/owner will get a better feel of the personality of the truck; knowing the actual performance of the rig can help prevent embarrassing mistakes - like running out of fuel on long trips (voice of experience - please don't ask); and gives better piece of mind about knowing the true capabilities of one's rig.
For what it's worth, I drive a 2011 Ram 2500 with the 6.7 Diesel. When I worked only 13 miles from home to work (50% hwy & 50% city miles), my MPG was between 12 & 13 MPG. In early August I changed employers and now drive 40 miles 1-way from home to work Mon thru Fri with 90% hwy & 10% city miles. My MPG has improved to 14 MPG and my hwy speeds avg 75 - 80 MPH.
My tires and wheels are "pull off's" from a new 2018 RAM Laramie 2500. The rubber is the stock Firestone Transforce street tread. The road noise is minimal and I'm sure they will get good long term miles, but they don't give me much piece of mind for off road use. Plus they tend pick up small pebbles and hold them in the narrow tread design. But I bought the set as new, (wheels and tires), for $1,000 to replace my older and original chrome steel rims and worn out Cooper tires, which I've still kept. Plan on buying new 2nd set of off road/winter tires that will have the new Snow Rating that will pass the new Nevada DMV law that states the only the New Snow Rated tires will be allowed to avoid chaining up this winter over passes like Spooner or Mt. Rose summits when snow conditions warrant the Chain Law.
Please keep us posted on your road trip travels and observations. Inquiry minds want to know.