Do you have your side mirrors set up so that you can see a bit of the side of your vehicle, or do you have them set wider?
It's important to learn how to adjust side mirrors properly as they are important safety tools, allowing you to easily stay aware of the road behind you.
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I'm a big fan of setting them out wider to cover the blind spots a bit better. I already know where the back end of the vehicle is, don't need to see it in my side mirrors.
Also, I don't trust in the mirror to show everything so I always look to verify.