Why are you building so many of your cities so they overlap?
This is a
very common question.
At the start of the game, your happiness cap for cities quickly ends up somewhere around size 5 or 6. That's as big as they're going to get for a while. Eventually that cap starts rising, and by turn 100 you might be having some size 10+ cities. By the time that you
actually care whether a city has 20 workable tiles or not, you're talking turn 200+. You don't care about anything that far in the future when you're settling your early cities. First priority is making sure they have some good source of food they can work
at size 1, preferably without even needing a border pop, to start growing immediately. Then you want them to have good general productivity at size 4-5, and you care about that a lot. You care a little bit about how productive they are at size 8, 9, 10. Anything past size 10 is so far off on the horizon that it doesn't matter.
And on the other side of things, there are some useful
benefits to overlapping cities. It cuts down on settler and worker travel time, and means you need fewer roads and it's easier to defend from barbarians. It reduces maintenance. It lets you swap improved tiles back and forth so whichever city will benefit most from that particular tile at the moment gets to work it. It can let you make sure that some cottage tiles are growing constantly, getting you fully upgraded towns faster that can then be handed off full-time to a city dedicated to using that commerce. It gets more tiles in a small area worked faster, which is useful if you're in a situation where your land is being squeezed by the AIs and you don't have enough room to just peacefully fill the land with as many cities as you feel like building.
In general, tile overlap is a good thing, not a bad thing, for early cities. This ties in to a broader general principle: small, fast gains are much more important than nebulous large gains happening at some far distant point in the future. Civ4 is a game about exponential growth, collecting immediate benefits and snowballing those little advantages into a big lead.
That's also why players recommend aggressively chopping down forests rather than preserving them until late game. It's why most buildings other than granaries are situational builds at best, because they take a long time to pay off and you can often get better short-term results by just taking more land. It's why placing cities where they don't need to wait for 10 culture and a border expansion before they become useful is usually a good idea. It's why land full of jungles (which require a
lot of worker-turns to clear and improve) tend to be lower priority than other areas when settling early, even though jungle often has lots of juicy-looking green grassland and resource tiles. It's a whole mindset: don't think about what will be good in 50, 100, 150 turns. Think about what will be good in 5, 10, 20 turns.