A bumper crop

For about eight months to a year, a new automobile remains clean and pristine. Something like a guest towel in the family bathroom. By then, automobiles have become more than a mere extension of the owner's personality; a person and his car become one. One doesn't buy an automobile, he marries it. At least there is a period of great devotion during which its body is washed with gentle Lux liquid and then caressed with a soft polishing cloth until it reflects the owner's beaming face.

During these so-called honeymoon months one finds nothing on the car to disturb its color harmony, its lines, or general design unless it is the somewhat vulgar display of a dealer nameplate pasted on the rear end. It is often a garish purple or gold emblem and advertises some remote dealership like ''Crazy Carl Chevrolet, Valvenok, Ohio.'' This one exception to the new, clean-machine rule is apparently excused as a sort of birthmark. An ineradicable beauty spot on an otherwise No. 1 bride. (Or groom, as the case may be.)

But after a car has aged about a year, romance fades. At least it does in America.

Americans, who have a wild compulsion to communicate, begin to paste up the car with signs, expressing personal views on every conceivable subject. These have become known as bumper stickers, regardless of where they are displayed.

This strange urge started right along with the first newfangled automobiles, as if something had to be said quickly before they exploded. At first, large banners were displayed, such as ''Pikes Peak or Bust'' or ''California or Bust, '' and then, inevitably, ''Finally Busted.''

Eagerness to spread messages continued, becoming even greater in the 20's and 30's mainly among car-owning teen-agers who covered the entire surface of their jalopies with dubious information like: ''Don't talk to the driver. He might be asleep.''

Eventually, through a process of evolution, the size of messages took on more modest proportions, while yet spreading to a wider sphere of topics. Reading a prescribed quota of bumper stickers may soon be considered the equivalent of a modern high school education.

All the issues of the day get argued out on the highways at 55 m.p.h. We drive in a frenzied stream of reading material. If it is true that Wellington said, ''The battle of Waterloo was won on the playing fields of Eton,'' the battle for America may be won on the passing lane of Interstate 75.

Politics is an easy target, things in Washington being funnier than things in Cleveland. Such statements about President Reagan as ''We've had enough - let Bonzo try'' have now replaced inferences about Jimmy Carter and the peanut crop.

One of the big issues on the highway is whether or not people should give up guns. Statements for keeping them are quick on the draw but underwhelming in their logic. ''God, Guns, and Guts made America. Let's keep all three'' presents a rather deplorable picture in relationship and might even suggest the driver preferred the reverse order. Some get more personal: ''My wife yes, my dog maybe , my gun NEVER.''

Slogans like that make me uneasy. Not because the person probably has a Smith & Wesson packed into his glove compartment, or that his eyebrows have grown together, but simply, why is a person who is so positive about everything else so wishy-washy about his dog?

Once when the traffic was bad, I spent nearly half an hour crawling along behind a sticker with the plea: ''Will the last American leaving Miami please bring the flag.'' It didn't say where to bring it. Presumably it would have to be at least north of Fort Lauderdale.

Southerners love bumper stickers. It gives them a chance to fight back when attacked by money-throwing Northerners each winter. ''Save the South. Teach a Yankee to drive'' points to one area of frustration.

Some of the stickers indicate a frustrated personality. These are the ''I'd rather be . . .'' stickers. I'd rather be flying. I'd rather be sailing. I'd rather be bowling. I'd rather, in fact, be doing anything else but what I'm doing, which is driving an eight-year-old automobile.

The more expensive the car, the less likely the bumper sticker. Expensive cars do not need bumper stickers, since they are a sort of statement in themselves. I have seen only one Rolls-Royce with a bumper sticker, and it had something to do with saving the whales. Perhaps it was a symbolic message for saving Rolls-Royces. When expensive cars do say something, it tends to have simple charm: ''Welfare Cadillac'' is one with appeal. Another is ''Stamp out Volkswagens,'' and this sentiment is occasionally extended to Hondas.

Many drivers show more offbeat slogans. ''Keep a cow employed. Drink more milk,'' ''Mafia staff car. Keepa you hands off.'' Or how about the one on a small foreign import: ''Don't honk; I'm pedaling as fast as I can.'' Some offbeat messages support certain professions. ''If you can read this, thank a teacher,'' or the more aggressive ''When you criticize farmers, don't talk with your mouth full.''

Cars traveling full of people in stained T-shirts and low-on-the-forehead hats, with a shaggy dog in the back seat, can surprise you with ''Christians aren't perfect, just forgiven,'' or a variety of folksy homilies such as ''Have you hugged your kid lately?'' Of course this hugging reminder is sometimes applied to dogs, wives, secretaries, and, perish the thought, even stockbrokers and Volvos.

These bumper stickers on the move are avidly read much the same way as the nearly forgotten, stationary Burma Shave signs, once stolidly placed at readable intervals along the highways. Who can forget those highly instructive verses, such as ''Half a pound - for half a dollar - lather just above the collar - Burma Shave''?

Will the traveling generation of today remember just as fondly things like ''God made a few perfect heads. The rest have hair,'' ''I brake for unicorns,'' or the one in large letters: ''I stop for elephants''?

The banners of communication, long may they rave.