The old timers' formula assumes a field efficiency of 82.5% -- in line with published estimates in the ag. engineering literature.
The theoretical capacity for a 9' swath at 4.3 mph = 9'*4.3mph/8.25 = 4.69 acres/hour. Rankrank1 has overestimated the field size and/or underestimated his ground speed.
Steve
If you read all my posts. I clarified that I had not done it in 50 minutes with my lowly Farmall h which is only 4.3 mph in 3rd gear. Still it was not all that much slower. The tractor I normally use is faster, I do not know the actual speed on it though as I have seen 2 different possibilities published for it. I need to lay out a distance and get my stopwatch out so I confirm its speed in the gear I use before I argue further.
You are also correct in that I may be overestimating my acres slightly. While I know what is in my field, I have relied on the neighbors info for his field. I need to either measure it or find a free google earth program to confirm his information to me again before I argue further.
Until then if we want to continue the pissing match my mower conditioner is actually 9' 3" cut.
1 acre = 43560 square foot
(4.3 mph x 5280 ft x 9.25' cut = 210012 sq ft/hour) then (210012 / 43560 = 4.82 acres per hour theoretical).
(4.6 mph x 5280 ft x 9.25' cut = 224664 sq ft/hour) then (224664 / 43560 = 5.16 acres per hour theoretical).
(4.9 mph x 5280 ft x 9.25' cut = 239316 sq ft /hour) then (239316 / 43560 = 5.49 acres per hour theoretical).
(5.2 mph x 5280 ft x 9.25' cut = 253968 sq ft/hour) then (253968 / 43560 = 5.83 acres per hour theoretical).
(5.5 mph x 5280 ft x 9.25' cut = 268620 sq ft/hour) then (268620 / 43560 = 6.17 acres per hour theoretical).
(5.8 mph x 5280 ft x 9.25' cut = 283272 sq ft/hour) then (283272 / 43560 = 6.50 acres per hour theoretical).
Of course these are theoretical 100% max efficiencies which are impossible in real world. You do not get a full cut each time (the efficiency here depends on overlap operator provides on each lap. The least amount of overlap improves efficiency). Also you are not always travel in straight line (efficiency here depends on pattern operator chooses to mow field - some patterns are much more efficient than others. Any time spent not cutting hurts efficiency. Some fields will be more efficient to mow than others due to their shape).
Again my original point to the OP was intended to be that even his lowly Farmall h should not be all that slow on his haybine. The OP's nice JD tractor should be even faster. The OP is mowing at 1.0 mph. Something definitely wrong with his haybine but that's all been lost now.