Regarding your premodern city size question—I don’t see a real constraint emerging from transportation speed. Here’s my reasoning using your figures: a city with N people needs N * 5000 sq. meters of land to supply it with food (assume the land can sustain production indefinitely). If this land is a disk around the city the furthest the food has to come is sqrt(N) * 40 meters. If the food is carried at 10 miles a day, the longest supply lines require transporting food for only ~2-3 days for a city of a million, or ~25 days for a city of a 100 million.
Of course this does not a lead to a clean limit because it seems to me there is no very simple limit on how long the food could travel before it is eaten… But especially the first of these (2-3 days) seems reasonable.
Now, as you note it is transportation cost that is key, not just speed. But this requires estimating many more numbers. What did your calculation / model look like?
Regarding your premodern city size question—I don’t see a real constraint emerging from transportation speed. Here’s my reasoning using your figures: a city with N people needs N * 5000 sq. meters of land to supply it with food (assume the land can sustain production indefinitely). If this land is a disk around the city the furthest the food has to come is sqrt(N) * 40 meters. If the food is carried at 10 miles a day, the longest supply lines require transporting food for only ~2-3 days for a city of a million, or ~25 days for a city of a 100 million.
Of course this does not a lead to a clean limit because it seems to me there is no very simple limit on how long the food could travel before it is eaten… But especially the first of these (2-3 days) seems reasonable.
Now, as you note it is transportation cost that is key, not just speed. But this requires estimating many more numbers. What did your calculation / model look like?