DUSTIN HOFFMAN'S TOUR DE FRANCE

Caravan of Promoters

The scene they witnessed included the caravan of publicity vehicles that precedes the race by an hour each day. Among the advertisers, whose fees pay more than half the operating budget of the Tour de France, were company representatives screaming the virtues of such products as chocolates, insecticides, paints, soft drinks, retirement annuities, breakfast foods, furniture and band-aids.

The scene also included, in the small towns through which the three- week race pauses as it travels more than 2,500 miles around France, the free lunches, concerts and folklore festivals like the one in Langon.

The setting of the race is to be combined in Mr. Hoffman's movie with the plot from the novel ''The Yellow Jersey'' by Ralph Hurne. Published in 1973, it tells of a British cycling veteran, now in his late 30's and retired to coaching, who is lured back to racing to help his protege win the Tour de France.

The protege falters but, by a wonderful coincidence, the veteran takes the lead when the first four finishers of a daily stage are disqualified for drug use. Can the veteran defend the yellow jersey, symbol of leadership? Will his efforts redeem an empty, dissolute life? Hoffman Expresses Hope

The novel is not doing well in the book stores, but this doesn't seem to worry Mr. Hoffman. ''They say, the better the novel, the worse the film; the worse the novel, the better the film,'' he remarked at breakfast in Pau before the race entered the Pyrenees.

Mr. Welland, a 50-year-old Briton who wrote the script for ''Chariots of Fire,'' was somewhat defensive. ''I cannot say I'm impressed by the book,'' he admitted, ''but we won't know what we have until I finish the first draft.'' He said he hoped to have this done by mid-October.

He also seemed undaunted by the prospect of making a movie about cycling, which is a minor sport in a major market, the United States. ''There will be a relevance to every other walk of life,'' he promised. ''Even if you make a film about Eskimos, it should say something to other people.''

Mr. Cimino, who directed ''Deer Hunter'' and '' Heaven's Gate,'' said he first began working on the movie in 1975, traveling with the Tour de France that year. ''These things take time,'' he explained. Scheduling Began in '80

Production, he said, was long controlled by Carl Foreman, who died last month. Shooting with the Tour de France was first scheduled in 1980 and nearly every year since.

Race officials have cooperated with the movie makers because they hope the film will create a surge of interest for the Tour de France in the United States, ''a consecration of cycling,'' as one official put it.

''The first thing I've got to do is to get a cycling coach,'' said Mr. Hoffman. At 47, he is about a decade older than most senior professional riders, but he dismissed this. ''The book is about the last moment of your youth,'' he said, ''and I think that's the way I feel now about myself.

''Actors say 'If I'm going to die, let it be on stage.' This guy says 'If I'm going to die, let it be while trying to make this curve.' I think I can relate to that.''