leaning fence post

I will venture a guess that you got one with a bad grain, a side cut. Other than taking it out an replacing it I don't think there is much you can do.

When buying the new one examine the ends of the post and try to buy one that is center cut. IE: the rings are about the same forming a bull's-eye.

Colbyt

Is the post leaning? Or is it warping?

If leaning, Put a stake in the ground, loop a rope around the stake and the top of the fence post several times. Stick another stake in the looped rope and tighten the whole thing up. Then give it a few more turns.

Tighten a turn or two every day.

When the post is straight again, strengthen the post's foundation.

Assuming the post is not bent, I would hazard a guess that there is a problem at the curb. Maybe they left a void when they put the road in. You may need to do some serious digging to find and fix the real problem. It is also possible that a water supple or sewer line has leaked and washed out that area. I hope I am wrong. If I am I would vote for trying the stakes and twisted rope trick, it can work.

Is it possible that a car ran into the post?

Good idea. If that is true, then I would believe that it will be relatively easy to fix since there will not be an underlying problem to correct.

Bingo! We just had Halloween.

I bet, though, if they ran into it, it would have broken. More likely they nudged up against it and PUSHED.