I happen to be one of those pesky engineers, a Registered Professional Mechanical Engineer with 50 years of experience.
Assuming the plows are in the same shape, and assuming it takes the same HP-Hr to plow an acre (too slow is less efficient as is too fast, there is the sweet rolling spot), you have decided to penalize the modern tractor.
Nebraska test 594 shows 3.2 gal per hr for a 720 diesel but test 605 shows 5.2 gal per hr. Much worse than you reported.
So using your numbers, with a D at 4 gal per hour, at about 10 hp-hr/gallon your designated field is using 40 hp to plow 1.9 acres per hour. 2.1 gal per acre. Or it takes 21 hp-hr/acre. The soil does not change between tractors so if the soil takes 21 hp-hr/acre, a tractor with 13 hp-hr/gal will take 1.62 gal per acre to do the same job. At your fuel prices it is about the same cost per acre, assuming your time is worthless it's a wash.
Now, it is critically important to run each tractor right at full load to make use of the maximum hp-hr/gal. They drop off quickly at lower hp. That's why new tractors have many gears. With the D you're stuck with first or second. If you're pulling 3-16's and it's belching and snorting and not at full RPM you'll find the efficiency is way down. Compare the slope of the HP-hr/gal curves for both tractors in the Nebraska tests. But you don't know because you don't have a tach. You'll know when you refill the tank, or run out of fuel before your calculations show you should.
I don't think you'll find the D can go as long between overhauls, however.
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