Car side mirrors. I spent more than a decade of driving adjusting them wrong. You want them so you don't see cars behind you at all in your side mirrors until they're transitioning off the edge of your center rear-view mirror. This gives you far more coverage of your sides, and every car is still visible in at least one mirror at all times. For me, where I put my seat, this usually means setting them as far out as they'll go.
[EDIT] I think I read about it in the context of correcting a common misconception or mistake, actually. Correct adjustment of the mirrors was described as watching a car leave your rearview mirror, and ensuring they show up on your sideview just as they're leaving the rearview—so if you can see cars on your sideviews before they're starting to slip off the edge of your rearview, you've got those adjusted too far in. For me, given how far back I put my seat, this ended up meaning I should adjust the mirrors all the way out in most cars. The idea is both to not have a gap in the sideview/rearview coverage, while maximizing side coverage within that constraint. So, if you can see cars in your sideview while they're still solidly in your rearview, that means you've got less side coverage than would be ideal.
Referenced in: https://www.caranddriver.com/features/a15131074/how-to-adjus...
(edited to include 2nd link)
1. When you set your side mirrors like this you are heavily relying on you rear view mirrors to see cars behind/around you in the other lanes. However your rear view mirror can easily get blocked by people or items sitting in the back of the car or large vehicles following closely behind who completely block your view of cars in the other lanes (remember side view mirrors will only show cars in your blind spot, not cars traveling in the other lanes that may be go significantly faster than you if you decide to change your lane).
2. Positioning your mirrors like this does not make all cars visible to you if you are on a road with more than 2 lanes and so you still must do a blind spot check by physically turning your head. This is because on a 3+ lane road and you are in lane 1, there may be a car in lane 3 moving into lane 2 while you are at the same time moving into lane 2 as well. Having your mirrors set so they only watch your blind spot you will not see that the car is moving towards the lane you are moving to until it’s too late. Doing a blind spot check you will be able to see a car 2 lanes over is trying to move to the same spot you’re trying to move to.
I like setting my mirrors so I just barely see the edge of my car when I look at the mirror. That way if there is a car behind me, their headlights aren’t constantly flashing at me from the mirror in my peripheral vision until I turn my head toward the mirror and then I can see his headlights near the edge of my car. I can see cars in the other lane but it doesn’t show cars in my blind spot (I still do a blind spot check). Another plus to this is that it’s easier to park backwards. The best of both worlds is also having a fisheye mirror on top of you regular mirror that shows you your blind spot but this is not a substitute for doing a blind spot check!
It’s not about seeing less. It’s just the fact that the rear view mirror is the most likely to get obstructed. I use the rear to mainly get a feel for the amount of traffic around me but I try to check it first then my side mirror then my blind spot before changing lanes.
Most people's side mirrors are filled with their own car which tells you nothing.