THIS SUMMER the average American tourist will spill the same McDonald's Special Sauce on the front of his Izod in Denver as he will in Des Moines. He will, with predictable regularity, fill his car with unleaded and his stomach with a burger and fries.

A growing band of adventurers, however, will not only sightsee but sight-eat, reviving from a tour of Disneyland with a Shields Date Gardens shake, completing a day in the Arizona desert with mutton and hominy stew at the Hopi Cultural Center Restaurant. Theirs has been an oral tradition in more ways than one; until now, they found the likes of Schwabl's in West Seneca, N.Y., or Melton's Barbecue in Rocky Mount, N.C., largely by word of mouth. But this year they have a bible: "Goodfood," the gospel according to Jane and Michael Stern, subtitled "The Adventurous Eater's Guide to Restaurants Serving America's Best Regional Specialties" (Knopf, $8.95).

It started with the Sterns' first book, "Trucker," published in 1975, for which they traveled across the country to photograph truckers and record their songs and stories. "We quickly found out that truckers do not know where the good food is," said Jane Stern, whose mouth had been set for a fringe benefit of months of good eating.

Eventually their editor pointed out to the Sterns that all they ever talk about is food, and it struck everybody that maybe they should write about it, too. From that came "Roadfood" in 1978, a coast-to-coast guide to decent meals for under $5 near highways, "a survival guide for travelers." But that still left them hungry to write about food that was not just cheap and decent and handy for travelers, but food that was local.

So this time around they changed the rules. They wrote about restaurants that provide a real taste of their region in food, style and price, and headed each chapter with a list of local specialties and where to find them: Cincinnati's five-way chili, the West Coast's Hangtown Fry, New Mexico's posole, Pennsylvania's scrapple and rivel soup and homemade root beer. Trying to limit themselves to three restaurants in each city, they left out ethnic restaurants (another book some day) and concentrated on those restaurants that had something really local as well as good--which meant they ate in five unusable restaurants for every one worth including. It also meant they felt compelled to leave out restaurants like Boston's Legal Seafood that have unimpeachable food but don't have regional flavor and local dishes.

For the last three years they have been on the road half the time, armed with source books by such writers as James Beard, Raymond Sokolov, Evan Jones and Time-Life and, most of all, a file of suggestions from the readers of "Roadfood." They scoured regional magazines and even their collection of old Duncan Hines restaurant guides. ("I have this horror," said Michael, "that in 30 or 40 years there will be Jane and Michael Stern cake mix.")

Michael, an art historian who studied filmmaking in graduate school, is the organized one and the one with the itch to travel, who maps out the routes. Jane, a painter, is the homebody who carries photos of their house as other people carry them of their children. Their method has been carefully worked out over the last dozen years of travelling: Go by instinct. "I like little skinny roads," says Jane. She might grab Michael's arm and wrench his gaze to what looks like a great restaurant. "No, it's too tall," he is likely to say, or, "It's too blue." And while they learned certain rules of the road, the crucial one turned out to be that there is an exception to every rule.

Take the Apple Tree Restaurant in Harrodsburg, Ky., an area that even the Kentucky tourist office has called "an absolute wasteland when it comes to food." The Sterns pulled into the restaurant one day because they were so tired of thinking about regional food that all they wanted was a plastic-food eatery where they wouldn't have to work. "It looks more like a California steak house," they subsequently wrote. But under the fake indoor tree with fabric leaves, in the shadow of window shades in Howard Johnson orange, they found plate-size corn cakes and lamb fries, fried pickled banana peppers and Kentucky's curious Hot Brown sandwich, not to mention country ham with red eye gravy and a knockout of a peanut butter pie.

All this was retold over a return visit to the Apple Tree one morning last month, when at 8:40 a.m., they were showing off their find to a couple of fellow adventurers. The waitress had hardly winced over the early morning orders for fried chicken, lamb fries and Hot Brown; at least Michael was ordering a true-blue all-American ham-and-eggs breakfast.

The Sterns were fortifying themselves for a mission: they had accepted a challenge to discover an unknown Kentucky countryside Goodfood place to show how it was done.

After the unorthodox breakfast, plus a short stop at a garage-restaurant for what turned out to be microwaved biscuits, they took a quick look at a small-town cafe menu and a cafeteria window, bought a canned-ham shadowbox for their editor at a bizarre craft shop and agreed that Paint Lick, Ky., sounded like just the place to find Goodfood.

Paint Lick's commercial district stretched across a half-dozen parking spaces. The town grocer pointed the group two doors down when they asked for a nearby cafe, and sure enough, it was the epitome of local eats: Paint Lick Rest. and Gro., with three women behind a long counter spooning up the $2.50 plate lunch of ham, pinto beans, real mashed potatoes with gravy, corn cakes, iced tea and strawberry shortcake made with fresh berries and cake "obviously homemade; you don't buy burnt cake," as Jane pointed out. Tomorrow's special was going to be fried chicken; last Friday's had been fried rabbit, and some days there was cobbler or butterscotch pie for dessert.

Of course the Sterns had known the Paint Lick Rest. and Gro. was going to be terrific as soon as they stepped in and saw all the down-home signs: the vinyl-covered tables in between the grocery shelves, the napkins wrapped around the silverware, the partitioned green Melmac plates and the local guys eating with their caps on, one of them calling to them, "If you aren't country folks, we'll make country folks out of you."

You can never tell about a restaurant, though, until you look at the menu, they have learned. Sometimes not even then, they added. Which brought them to the subject of their favorite menu-ese--undoubtedly the germ of another book. In Baltimore the Sterns found a menu promising "rock of lamb." "And they were right," Jane chimed in. Then there was the menu that offered steak "ordered to your likeness."

While they were researching their book their day started at 5:30 a.m. and ended when they dropped into a hotel around 8 p.m. It was a rare day when they ate less than five or six meals; thus they got very good at secretly wrapping food in napkins so they wouldn't hurt the feelings of their waitress. Sparsely populated places like Wyoming were a relief; "In Wyoming we are lucky if we get two meals a day," they said.

Suburbs are rough going for anyone seeking regional food; the Sterns couldn't find a single possibility in their home county of Fairfield, Conn. And four whole states never made the book. Hard as they tried, the Sterns couldn't find any distinctive local food in North or South Dakota, Montana or Delaware. In South Dakota there was "great Czech food" but nothing regional; in Delaware the food they found was indistinguishable from Maryland's or New Jersey's. They didn't include any ethnic food that hadn't been changed enough that its country of origin wouldn't recognize it, but St. Louis' toasted ravioli made it, and three pizzas: New Haven's white clam pizza, Memphis' barbecued pizza and deep dish Chicago pizza.

The Sterns caution that Goodfood is not always good food. They still haven't figured out the rationale behind Maryland's fried hard-shell crabs, or how to eat the crust and the meat while avoiding the hard shell. And they allow a lot of anthropological latitude for the likes of Cheez Whiz on the Hot Brown and for Jell-O salads, as well as for bizarre menus. "We felt we could allow them to have stupid stuff on the menu as long as they had enough good stuff," said Jane.

After years of diners, luncheonettes, oyster bars and boarding houses, they still are irresistably drawn to driving miles over country roads to check out a gas station that is said to cure its own hams, and can't pass a cafeteria without checking the daily specials. Jane frets over restaurants in the book changing, and has long telephone conversations with them whenever such a rumor reaches her ears.

Now the Sterns want to stay home for a while, mostly to work on a cookbook they tentatively call "Square Meals," with recipes for good old out-of-fashion foods such as tuna noodle casseroles, dried onion soup dips, nursery food and vanilla butter. They are also doing a book--half cookbook, half shopping guide--with Manhattan's Dean and DeLuca specialty store.

The inevitable question is how two people write together. "We don't have a four-handed typewriter," Michael answers for the umpteenth time. When they leave a restaurant they talk about it while one drives and the other takes notes. As for the actual writing, "each takes the restaurants we love, and if there are any left over we don't write it up." They are sufficiently in tune with each other that once the reviews are written, they say, even they can't tell who wrote which.

Lately they have added a Hartford Courant restaurant column to their workload, and they hope to keep doing "Goodfood" revisions as long as there is good food. Would they like to turn tables and do a book on France? Well, they don't speak French, but on the other hand, "In many parts of the country we don't speak English," answered Jane.

Ideas for future books are infinite, they say: They could do a whole book on wieners. (In Rhode Island you order "wieners up the arm" for a large group, and the waiter literally stacks the buns up his arm, then the wieners in them, and finally the sauce. "The trick is not to get the one near his armpit," warns Michael.) Or on barbecue sauces. (In Texas the barbecue is accompanied by a jalapeno and a half stack of Saltines, hacked with a knife and served in the bag.) They collect food lore and history. (The fried clam was invented by Lawrence Woodman in Essex, Mass., July 4, 1916, once he figured out how to fry something that comes in a shell. Obviously the Maryland crab fryer found another solution to the problem.)

In the meantime, food writers galore are combing the byways for regional recipes and regional lore in the great exploration of American Cookery. Doesn't worry the Sterns. They delight in sharing a "sleeves-up mentality" with Calvin Trillin, for instance, explaining, "He whets the appetite and then we tell people where to go."