Hit & Run," the low-budget, lowbrow car chase comedystarring Dax Shepard, Kristen Bell, Bradley Cooper and Tom Arnold, is a strange, but strangely entertaining combo of drag racing machismo, slapstick silliness, raunchy riffs, politically incorrect rants and sweet nothings.

Revving up its R-rated engines, then detouring for relationship repairs, the sincere and the absurd work in fits and starts. Though "Hit & Run" isn't consistent enough to put it in the league with car chase/rocky relationship classics likeRobert De Niro and Charles Grodin's "Midnight Run," it's certainly a contender in this summer's guilty pleasure race.

Shepard co-directed, with David Palmer, and wrote the story in which he plays a former bank robbery getaway driver now living in small-town safety under the witness protection plan and an assumed name — Charlie Bronson. Life is sunshine and lollipops until new girlfriend Annie (Bell, who is Shepard's real-life fiancée) has a shot at her dream job at a major university. The only downside is that it would require going back to the scene of his crimes, in this case Los Angeles.

Driving Annie there would be risky regardless. But Charlie's decided to take the old getaway car out of mothballs, a 1967 souped-up Lincoln (one of Shepard's real cars; there's a pattern emerging). It makes flying under the radar much harder and is only one of many complications that will beset the couple. Indeed, their relationship is threatened at every turn as cars are mangled, the truth is twisted and Charlie's dark secrets get spilled.

Helping mix things up are a series of comic sideshows, most of them conducted by Shepard's circle of friends, or at least those willing to work for scale. Randy (Tom Arnold huffing and puffing delightfully) is Charlie's federal marshal minder. More baby than babysitter, he's a gun-dropping doofus who is especially dangerous when he's behind the wheel, which in this movie is a lot.

But the serious trouble starts when Annie's pecs-flexing ex (Michael Rosenbaum) Facebook-friends the guy Charlie doubled-crossed. That would be Alex Demitri (Cooper), who swings wildly between sensitive and brutally insane, his bleached-out dreadlocks helping diminish the usually suave power of People's sexiest man alive.

Meanwhile, Shepard just keeps piling on more bodies. Joy Bryant, who plays Shepard's wife in the TV drama "Parenthood," turns up as Alex's girlfriend. Charlie's estranged dad (Beau Bridges) checks in with a bone-crunching cameo, Annie's boss (Kristin Chenoweth) is another piece of work and a bunch of other comic actors drop by.

As promised by the title, there is a lot of car chasing on the road to L.A., although it seems more of the donuts-in-a-parking lot variety — tires squealing around tight circles. But Shepard's character spends most of his time dealing with everyone else's issues. As it happens, Alex's anger at being double-crossed is nothing compared with Annie's, who keeps reeling as new details about Charlie's past emerge.

The writing is more disciplined than the directing, which gets rough around the edges the more the action spins out of control. The benefit of having friends fill the film, besides the sheer talent, is that it makes for good chemistry on screen; not surprisingly, the best is between Bell and Shepard. From the opening moments that find Charlie and Annie still in bed, the tone is set — playful sweetness, rather than steamy sex, as he tries to shore up her ego. The couple has said Charlie and Annie's relationship looks a lot like theirs and you believe it.

The comedy, however, comes from any number of sources. There is a good bit of slapstick, most of it courtesy of Arnold, whose marshal is about as hapless and hopeless as the Stooges' Curly. At other times the humor comes from unexpected quarters, the "surprises" as likely to elicit groans as guffaws since this is where the gross-out factor is in overdrive. And finally, there is the funny that is mined from the friction that accompanies any relationship.

This is Shepard's strong suit and it is where the film is at its best as the actor uses his perpetual talkie-deadpan to tackle hard truths. Whether Charlie is arguing that teasing is just a ruse for real issues or trying to diffuse Alex's anger over a humiliating prison romance, Shepard has an eccentric way of mixing the farce and the facts. It serves to make Charlie as endearing as he is frustrating, which definitely comes in handy for the times "Hit & Run" goes off track.

betsy.sharkey@latimes.com