What's the tone of "hum" by Jeff Gundy? How does he use it through the essay

The car purrs along at 72, engine noise lost in the whisper of wind and the music. I love to hit the Volvo's cruise control and turn up the stereo and roll along alone, checking the mirrors, the gauges, letting my thoughts go long and loose, knowing it'll be hours on the road and that I can stop for coffee and gas and to pee whenever I feel like it, that I have credit cards and cash and extra oil in the trunk just in case.

Between my mushy tree-hugger tendencies and my friends buying gas-electric hybrids I'm a little guilty about the S90, which gets 27 miles to a gallon at best. But because it had been badly mistreated we got it way cheap, and having worked it all over with my son I know it better than any machine and most of the people that I've ever met. It still has its quirks and mysteries, trembles a little at idle, uses too much oil, shakes sometimes when you hit the brakes. But at 70 it rides like a dream, smooth and steady, ready to carry me down the longest road and into the deepest sunset.

I go days on end and never get in a car, walking to work as I do, so I love to put my bag and books in the back, arrange my coffee mug and case of cd's and map and head off to play poet somewhere. It feels good to call off classes, give the students some token work to do, get up earlier or later than usual and hit the road, knowing I'll be greeted and fed, paid attention and shown to a good bed.

But all that is just backdrop for the pleasure of humming along on whichever interchangeable midwestern interstate it is. I emerge from driving reveries not knowing where in the world I am, except that I am in the small private space of the car, within the whirring, complicated, efficient machine that bears me along the way I used to think my soul was carried around by my body.

A few hours in the car and I drift free from my body, cushioned in the leather seat, the temperature steady, nothing to do besides keep a hand on the wheel and stay between the lines. I snatch every glimpse I can of what's beyond the shoulders of the road, among the trees, in the sky.

The car is the body and the body is the soul and the soul is no longer inside the body but floating, flying, slipping through the window to lick at the air like a golden retriever, its wisps and tatters trailing wildly in the wind of passage.