Grit,
That square body your son has can probably do pretty well in the tire diam too!
But with that said, if one wants to get real on tires, my best luck has been with cooper's. The AT3 is a winner for more than one person who's had them. Including some 245 75 16s I had on a rwd 2500.
I've have transformed HT on a work van, got some 70k Mike's out of those doing hwy driving 50k seems more common if you doing a mixture. Not great in snow. Rain and dry roads fine.
Toyo have been good.
Michelin and Good Year have been a mixed bag. My current 1500 came with GSA, horrible in rain. No idea about snow. Worst yet probably. I have duratrac now. A bit noisy on dry roads, great in snow, rain. I should have have gone with the AT3.
Not made anymore, michilen xde m+s if you want a hwy traction in a 19.5 size. Otherwise, skip this brand.
As I said initially, tires are like truck brands and other things. Most in reality are good and will do what they are designed to do. Take them outside of that spec, they suck....
Mileage will depend upong driving style. All hwy, you'll get 20-30% more miles than I will doing city driving. Loaded less than unladen.
Choose you poison, enjoy life.
Say high to tim on Gary Lane if you know him.
Marty