ifyjo said:
Question: Which is better overall [if possible to calculate], mining the area around a city, or irrigating it?
If it is easier to answer this then do so: which is better for finishing making things [wonders] faster, mining or irrigating?
To be more precise, at the start of the game you want your cities to grow till they hit their growth cap, once they are there, you need exactly enough food to keep the city from starving (so irrigate only enough to achieve this) and maximize production and commerce.
ifyjo said:
Lastly, if the answer is different for plains please say so
If your city is on all planes, then you should obviously irrigate it all, except for 2 tiles, so that the total food will be 24 (enough to support 12 citizens) with maximum shields per turn.
When you have railroads, the food from the irrigated plains go up, so then you can afford to switch some more irrigation to mines.
Keep in mind also that in despotism, there is a penalty that removes one from each tile that has 3 or more. Irrigating grassland will not do anything here, because the 2 food from grass turns into 3 with irrigation, then the penalty removes 1 and turns it into 2 food again.
Of course, when a tile already has 3 food (or more) naturally (floodplains, or because of a food bonus resource) then the penalty has already been applied so irrigating such a tile under despotism will help.
Because growth is important in the early game, you should irrigate any tile that can break the penalty. But at later stages (even if you stay in despotism) Irrigate only enough to keep the city stable at its maximum size (usually size 12)
EMan said:
......And, make good use of Civil Engineers & Scientific Great Leaders.
I rarely use Civil Engineers, by the time they come available, it usually no longer matters.