“What We Find on the Road” is a dramatically dull indie roadtrip dramedy that reaches for “coming of age” and strains itself getting there.

The film begins with mystery and promise, drifts into predictable and undramatic and doesn’t really rally to become anything more than a scenic screensaver as it follows an 18 year-old driving a ’68 Dodge Polara from Cape Cod to California…by way of Tennessee (!?) and Texas (!?).

But if you’ve ever nursed an overheating, engine-knocking beater on a long drive and if you’ve never been to the Grand Canyon or seen a “road picture” in your life, it may have a little something to offer. Very little.

It stars young and still unknown bit player Finn Haney and was scripted by his father Bill, a documentary filmmaker (“The Last Mountain,” “The Price of Sugar” and “A Life Among Whales”), producer and sometime screenwriter.

Haney-the-younger plays TC, who just turned 18 and is looking at the collegiate future when an aged pot-bellied biker shows up at his door, asks his name and confirms that birthday.

“You old man wanted me to give you this,” he growls, and leaves the kid a set of car keys and a Vallejo, California address. The car? It’s at an old Cape Cod repair shop down the road, where Bill presides.

What’s rule #3 when you’re making an indie film, kids? Write a chewy part with a pithy speech or two that you can use to talk a “name” into taking that role. Here, that pays dividends as the great character actor Paul Guilfoyle (“C.S.I,” “The Good Fight,” just seen in “Arthur the King”) plays the sage of the sparkplugs mechanic who has stored that ancient “Blue Biscuit” convertible for “The Hammer,” TC’s estranged father.

The Hammer had a reputation — “rock and roll” and drugs and trouble with the law — which is why TC’s unseen mother wants nothing to do with him. But Bill leads the kid through the bring-the-383-cubic-inch-engine-back-to-life in a montage, and gives him a flashlight, “a c-note,” car advice and life advice before the boy sets off to meet a father he never knew.

“Start saving money for gas, it’s a guzzler.” And “You’re being perversely tested by your old man — 3000 miles in a beat-up old bomber to find your father, who hasn’t exactly nailed the fathering business.”

And with that bit of wisdom — with no mention of updated registration, insurance or title, a wonky radiator and a mysterious steel box welded into the trunk — TC is off, to be “tested” along the way, we figure.

Of course bestie Jake (William Chris Sumpter) shows up and demands to go along, as TC hasn’t ever driven on the highway before, ditching his parents’ Volvo wagon at the garage as he does.

The screenwriting problem-solving logic leaves a lot to be desired in this narrative, from the lads not telling their parents (While licensing and registering a car? While ditching a Volvo?) to their first stop, a pointless “upstate New York” visit to relatives that adds nothing to the narrative, no colorful supporting characters or performances of them. That first breakdown/traffic stop is a “Tennessee” cop whom they somehow encounter between Cape Cod and Vallejo via upstate New York in an overheating antique.

If your script’s not as clever as “Doc Hollywood,” you can’t get away with sloppy geography (D.C. to Beverly Hills via South Carolina in an antique Porsche, in that film.).

We’re treated to generic “road food,” “sleeping in the car,” radiator issues and the ongoing mystery of what might be in that box, which provides suspense with every traffic stop. None of it adds up to much that holds the interest.

His friend leaves and an Irish divorcée (Katherine Laheen) with her dog and a broken-down pickup take his place. The Grand Canyon is visited and bullies are brushed by as TC heads for that fateful meeting with the father who may have stashed drugs, drug money or who knows what in that box in the trunk of a car he’s sentenced his son to deliver to him cross-country.

There’s just not enough drama, charm, whimsy or angst to make this picture live up to those earthy early “change the plugs, alternator, tires and hoses” scenes with Guilfoyle. The insights are generic, the performances generally drab and the payoff’s more of a bust than a catharsis.

It’s scenic, and the traveling-far-in-an-unreliable-car makes “What We Find on the Road” relatable to a lot of us. But we don’t need a movie to remind us how tedius long drives generally are — with or without breakdowns, nosy police or fiesty Irish damsels.

Rating: R, profanity

Cast: Finn Haney, William Chris Sumpter, Katherine Laheen and Paul Guilfoyle.

Credits: Directed by Chaysen Beacham, scripted by Bill Haney. A Dada Films release.

Running time: 1:34