To many US families planning summer vacations, a key question is likely to be: Are gasoline prices headed back up again?
In trying to answer the question, energy experts first scan the entire petroleum supply chain -- stretching from the oil-exporting Mideast, major multinational oil companies, and US wildcat drillers to local filling stations.
Then the experts reply with a maze of projections, including:
* Today's world oil glut will end within two months.
* The glut will continue for another three to 10 years.
At one extreme, oil industry analysts say they foresee supplies tightening over the short term. Some even warn that American motorists may turn the corner soon and find themselves at the end of stall-and-crawl lines leading to gas pumps where sales are rationed.
At the opposite extreme, some experts see a smooth ride for motorists -- who like $1-a-gallon gas -- but a long, rough road ahead for suppliers.
Oil industry and government economists say the gasoline price plunge already is causing severe adjustment problems for the petroleum industry, which based its long-term plans on continuing price rises. Everything changed, they say, when the price of gasoline began its long slide from a March 1981 average of $1. 42 a gallon (for unleaded regular) to $1.28 in March of this year.
Prof. James Sweeney, director of Stanford University's Energy Modeling Forum, says he expects ''a small upward trend in gasoline prices this summer.'' But he warns against ''watching these short-run price fluctuations.''
Government policymakers, businessmen, and individuals about to buy a car or insulate a home, he says, should avoid ''the overemphasis on month-to-month changes as harbingers of the future, because there is so much that can happen that it's easy to be misled.''
Dr. Sweeney says the forum's World Oil report, released last week, concluded that ''we should expect some continuation of an oil glut for perhaps five or even 10 years. . . . But around the end of the decade perhaps we would expect to see . . . sharply rising world oil prices, even without any major supply disruptions. So we have a window of perhaps five years or so in which to deal with world energy problems.''
US Department of Energy spokesman Jay Vivari is more cautious. He warns that with so many variables, ''it is simply far too difficult to speculate with any reliability.''
One key uncertainty, Mr. Vivari points out, results from the dramatic strides Americans have made in reducing energy demand through such changes as driving smaller cars and insulating buildings better. With such changes, he says, suppliers have no way to estimate ''what are adequate stocks today, given today's lower demand.''
Some analysts warn that the US risks running short by draining oil inventories well below normal levels. They say they foresee supply problems developing once the recession ends and demand increases.
One Houston stockbroker specializing in energy issues says, ''Now, just when we are faced with the onset of the summer travel season, many delivery points around the Midwest are out of regular gasoline. . . . Some of my people are telling me that there will be gas lines again this summer.''
Even without any dramatic supply disruptions, he warns, ''Anywhere east of the Rockies, you could have some spot shortages at any time.''
This Houston broker sees the industry's sharp reductions in oil inventories as a threat. ''Fly over the Houston Ship Channel, and you'll see all those empty oil tanks. The result is that when we get a surge in demand, then we are right back into that same old crisis mentality, and there'll be a run on gasoline stations again.''
But Vivari and others speculate that today's lower inventories may prove adequate to handle demand that has been permanetly reduced by conservation measures.
Last week's report from the Independent Petroleum Association of America's Supply and Demand Committee backed up the view that demand will continue to lag behind supplies.
IPAA spokesman Peter Wellish points out that due to conservation and new energy habits, ''even as prices have dropped, we have not had any great increase in petroleum usage.'' The IPAA report, he says, concludes that ''energy consumption will stay down'' -- decreasing 1.2 percent in 1982. This would follow declines of 2.6 percent in 1981 and 3.8 percent in 1980.
Declining demand, say Mr. Wellish and others, will hold down gasoline and other energy prices. Robert Pindyck, professor of applied economics at MIT, says that ''if there are no surprise events such as disruptions in the Middle East that would affect petroleum supplies, prices should stablize around where they are now.''