In a time of runaway inflation, one of the hottest items in Paris is a $20 tour that takes in the Louvre, Place de la Concorde, the Sorbonne, Montmarte, the Eiffel Tower, the Arc de Triomphe, the Champs Elysees, the Madeleine, Place Vendome, the rue de la Paix, the Opera, Notre Dame, the Bastille, the Pantheon, Saint-Germain-des-Pres, etc., etc.

To a Frenchman, the $20 tab is a mere drop in the Seine, particularly since the setting is Paris. Still, if the 20 bucks seems a trifle steep, the same company will show you Paris by night for only $14.95. Or else send you cruising down the Seine for next to nothing. A mere $4.10, to be exact.

These and other excursions are among hundreds being offered by that granddaddy of tour operators, Gray Line. With 174 member organizations worldwide, 1,501 daily excursions and more than 20,000 employes, Gray Line ranks at the top of the heap. Its buses roll round-the-clock, from Bangkok, Thailand, to Bangor, Maine.

Founded in 1910, Gray Line carries 40 million passengers a year, an operation that began with a single Mack-truck bus, a part-time mechanic and a driver doubling as a lecturer-guide. No air conditioning, no cushioned seats. Just a bouncing bus with President Wilson along on the inaugural push. The scene was Washington, D.C., the fare was a dime.

Seventy years later, at a time when the price and availability of fuel have affected motorists' driving habits, the company's 8,500 motor coaches are racking up miles in 56 countries around the world.

According to Gray Line, their excursions rank among the best buys in travel, with half-day excursions averaging $6 to $11 per passenger. Even in expensive Manhattan, Gray Line provides an all-day tour -- bus ride included -- for a flat $17 for adults and $13 for children.

Rated high on Gray Line's popularity poll are Paris, Rio, Hong Kong, Bangkok, Amsterdam, Dubrovnik, Tokyo, Tel Aviv, Guatemala City and Mexico City, while here at home the leading contenders include the Grand Canyon, Zion, Williamsburg, Disneyland, Disney World, Universal Studios, San Francisco, Manhattan, Washington, D.C., Denver, Monterey-Carmel, Charleston (S.C.), Nashville, Seattle, Vancouver-Victoria, the New England states, Hawaii and Reno-Tahoe.

While Gray Line is a well-oiled operation, occasionally it's been known to develop a squeak. In Nassau, for example, the franchise was passed from one operator to another due to poor equipment and tours that failed to meet Gray Line's high standards. A similar situation developed in El Paso with a resulting switch in companies.

Headquartered in Manhattan, Gray Lines is composed of dozens of bus companies. While individually owned, each operates under the Gray Line banner under policies set down in New York. Obey the rules or get out. It's as simple as that.

Although she's the gray old lady of the bus beat -- punctual, dependable, affordable -- the organization does provide its bright moments. As, for instance, outside Phoenix, in the old shoot-'em-up town of Rawhide, gunslingers are hired to "capture" the bus. Even take a "prisoner." Once, though, the gag backfired when a new driver, unfamiliar with the Gray Line script, geared up and made an effort to escape.

Grossing $100 million a year, Gray Line engages limousines, narrow-gauge railroads, cruise ships, helicopters, paddle-wheelers, tramways, Jeeps and sleighs in addition to its motor coaches, carrying more passengers altogether than the top three U.S. airlines combined.

In Las Vegas, Gray Line ranks as the desert spa's busiest operator of nightclub and casino excursions. From Washington, D.C., it conducts 15-hour tours to an amusement park in Williamsburg, Va.; from Toronto it takes in Niagara Falls with stops enroute at Tivoli Miniature World, Niagara-on-the-Lake, Welland Canal and Queenston Heights. Tours emanating in Florida move off to Walt Disney World, Sea World and Circus World. Out West in Arizona, other buses roll into Mexico and the Wyatt Earp-Doc Holliday country of Tombstone and Bisbee.

Like Britain in her days of wine and roses, the sun never sets on Gray Line's empire. Her buses move out day and night. One can be assured that Gray Line passengers, no matter the hour, are firing their Instamatics while driver-guides let fly with their spiels.

Gray Line does business in such offbeat global spots as Raratonga in the Cook Islands and Ise-Shima National Park in Japan, to say nothing of Dhakshinkali, Dhulikhel and Simbhanjyang in Nepal.

In Indonesia, one may hop a ride with Gray Line from Jakarta to Bogor, Bandung, Yogyakarta, Surabaya, Trowulan, Trestes, Mt. Bromo and Denpasar, a 13-day tour tagged at $900 per person. Including full board, it comes out at pennies under $70 a day, or less than a good hotel room either in Los Angeles or Manhattan.

In Taiwan, $84 gets the rider a two-day bus and train tour to Sun Moon Lake with an overnight stop at Sun Moon Lake Hotel and a visit to the Aborigine Village and Peacock Garden. Or there's a three-day excursion that takes in Hualien and Taroka George before proceeding on to Lishan and Taichung. Up for grabs at $168, the price is all-inclusive with the exception of meals.

In Greece, other Gray Line tours operate to Cape Sounion, Delphi, Crete, Rhodes, Mykonos, Heraklion, Santorini and Patmos. In this case, Gray Line boards its passengers on cruise ships, picking them up by bus at the various island stops.

Gray Line passengers visit crocodile farms in Thailand. Maori villages and the thermal region of New Zealand, the Mayan ruins of Mexico, the Valley of the Fallen in Spain and Anne Frank's house in Amsterdam.

The 70-year-old company is involved in package tours, rail and air transfers, rental cars, limousines and half-a-dozen other services. During 1981, Gray Line plans to extend its operation into Casablanca, Munich, Frankfurt, Belgium and Saipan. Recently, the organization signed up a 110-year-old company in London as well as long-established Appian Lines of Rome.

In the United States, Gray Line's biggest market is Los Angeles, with San Francisco running a close second, followed by Manhattan. Steering the entire operation is 53-year-old Patrick R. Sheridan, a graduate of Notre Dame, TWA, Cunard Line and Matson. By his own count, Sheridan has 19 "bosses" on the board. Still, Sheridan lets everyone know that he's the guy in the driver's seat.