I was able to locate a ratio of .33 hp/acre, in an old pamphlet that was supplied from the dealer, with my 1951 Ford 8N. That says, that little 28 hp beast was designed for an 84 acre farm, by itself. My neighbor was well-to-do, and only ever used it on a medium sized garden. These days, I don't think you will find the ratio anywhere. If you do, it will certainly be a lot higher, as everyone thinks "bigger is better". Also, everyone has a lot less free time, since the advent of things like tv, computers, cell-phones, and micro-wave ovens. Interestingly, the prices are also penciled in that old pamphlet, with the tractor listed at $1435, and the 2 x 12 plow at $194. My long deceased neighbor bought them brand-new back then, prior to his widow selling them to me (with all the paperwork), about 25 years ago. Back then, my granddad could not afford the Ford, and went with a competing 2-plow tractor, the John Deere model M, when he replaced his own team of horses. The M was a little smaller (20 hp), and I know that he maxed out at about 65 acres with it (.31 hp/acre), so he nearly matched the Ford in output. Enough to make up the cost difference anyhow I suppose.
Also, for some more ancient numbers: Back when my other granddad worked 50 of his 70 acres, he used 2 Farmalls (F-13, and F-20) for a total of 33 hp, and a ratio of .66 hp/acre. Although he was using more power, his tractors were bought used. They were considerably older and less reliable than the 2 machines, purchased brand-new, that are mentioned above. That is a good point to consider when buying tractors today. In general, the used ones will save you some money up-front, but may cost you more, in the long run, due to reliability issues. That is of course, unless you find some real good ones, as I have managed to do a few times, over the years. The old M is worn to a frazzle due to all the work it has done over the years, and the Cub ain't much better. The C has been fair, while the 8N has been as reliable as the modern tractors.
Now for some of todays numbers: On that 70 acre farm (which I remotely operate now), that old John Deere M still purrs like a kitten (a loud one given the big 2-popper motor). With that, along with another, modern 20 hp John Deere (model 770), I maintain 10 cleared acres on a 70 acre farm. The rest is mature hardwoods, or has reverted to thick cover. The 770 is used probably 10 times as much as the M. This gives us a ratio of 4 hp/acre. Certainly a lot higher than the old granddads got away with. On my own home, 35 acre farm (25 acres cleared), I match that ratio (4 hp/acre), with 101 hp total, split between 4 tractors. These include the 28 hp Ford 8N, a 20 hp Allis Chalmers C, a 10 hp Farmall Cub, and a modern, 43 hp, John Deere 4120. I would guess that the 4120 gets aroud 50% of the total use, with the Cub and 8N each around 20%, and the C around 10%. In my opinion, non-cleared, non-maintained acreage should not be included in this calculation.
One thing is for sure, I would never recommend a single tractor for anything much over 5 acres. That means putting all your eggs in one basket. Any tractor, new or old, can break down at any time. Your wife, friends, or kids, will often have nothing to do. If you get stuck, there is no easy way out. Also, almost every job can be matched for size. The little Farmall Cub is perfect for cultivating the garden or small corn fields, where the 4120 would be grossly oversized. Few tractors work better for pulling logs out of the woods than a trike like the C because of such super forward visibility compared to a wide-front tractor. The 8n is just right for hauling firewood out of the barn, or pulling a 2-row corn planter. I would be lost if I couldn't match the size of the job to the size of the tractor. Another thing you will notice with multiple tractors, is less total breakdowns, because no one gets used as much.
So, bottom line, here you go for my family anyhow: Old Days New Tractors: .33 hp/acre
Old Days Old tractors: .66 hp/acre
Now Days combination New and Old Tractors: 4 hp/acre
