I've got that chart in my 2020 book too, but it doesn't tell me anything. Strange thing is looking at my tach/speedo the one on the tractor, which is a replacement, says almost 7mph in 5th gear at 1500 rpm. 6th gear is 10+mph on the speedo. I was looking at tach/speedos on line so who knows what they are, as well as the one I've got.
What I just did was measure out .5 miles with the P/U. Drove the tractor over the same .5 mile in 5th gear. Takes 8 min 44 sec on the stop watch. Then drove back in 6th gear. Takes 5 min 58 sec. Now I'm trying to wrack my brain to figure out how to convert min/half mile into mph. I used to be good at math, but I've lost my mojo with the age.
I did figure if I'm moving 3mph and cutting 7' at a whack and it takes me 3 hours that's 7x3x5280 sq feet, divide that by 43500 or so and I get 2.5 acres mowed in 3 hours. I know the field is larger than that. It's about 8-10 acres. So 7mph may be about right for 5th gear because that is what I was using.
But if someone can steer me from 'min/half mile' into m/h that would be great.