When I'm in the car I'm really particular about what "number" the volume of the stereo is on. It either has to be on an even number, or a multiple of 5. The "multiple of 5 rule" trumps the "even rule" though, so for example, it's preferable to have the stereo on 15 rather than 14 or 16 even though they are even numbers, lol.

Even if DH is driving and he changes the volume it will really bother me if it doesn't follow my "rules" until I change it. He just laughs at me now.
Not a good rule, Mylitta (and others who also do this). Prime numbers are best for the car stereo, ideally ending in three or seven, but most odd numbers are allowed. Even numbers are permissable, but squares are best, even though they are not prime (eg 4, 16, 25...). HOWEVER, multiples of three and, to a slightly lesser extent, seven are not allowed. So 9 is out, even though it's a square. Both digits the same are also desirable, but not necessary and NOT if they break the multiples of 3 or 7 rule.

Favourite car stereo volume numbers, in order, are: 13, 17, 16, 7, 11, 19, 22. Yes, I do have a long commute to work each day and yes, this is the most important thing in my life.