Congrats on the new 638 ...
I have one of the older 649's, and although it required repair right out of the box ... cylinder fitting and rebound slide problems ... that was back when S&W was having their spotty quality control problems in their revolvers in the 80's and early 90's ... After the head armorer corrected the problems it's been a terrific little snub gun. Just "heavy" ...
So, I bought one of the 642-1 Centennials, and it's a VERY good little snub gun. I especially like that the latest aluminum guns are rated for a lifetime of +P use ...
My 642-1 "required" some minor attention by the head armorer, who's a S&W revolver armorer (I'm not, only on pistols) to remove some "burrs"
in some engagement surfaces, and we tested a couple of different weights of Wolff rebound slide springs to find one that reliably returned the trigger as fast as I wanted to be able to shoot ... and then all it required was some of the bright neon orange day-glow sight paint, so my aging eyes could see the front post in the early morning sun ...
And I simply love this little revolver ... It slips into almost any pocket, and can be taken places ... comfortably ... that my other off duty weapons aren't easily carried.
I think you'll find an ankle holster a very easy way to carry your snub within easy reach while behind the wheel, by the way ... I don't favor them for anything else, but when behind the wheel, they're easily reached by simply moving your weak side leg back toward the seat. They do, however, subject the guns to more debris, depending on your pant cuffs ...
By the way, when I used the exposed hammer of the 649 to make SA shots, I often surprised a lot of the newer folks with how accurate the little J-frames can be. These are very underestimated guns when it comes to accuracy.
Enjoy ...