- Salt Lake Valley motorists paid an average 88.7 cents per gallon for unleaded gasoline last month, 85.5 cents for regular, 98.1 cents for premium unleaded and 98.1 cents for diesel fuel.

- At 88.2 cents per gallon, Holiday Oil had the lowest average gasoline prices locally in December while Rainbo Oil Co. was the volume sales leader.- With a 15.23 percent average, Salt Lakers are well below the national average - 27 percent - in the percentage of premium fuel they buy for their vehicles and not even close to New York, where 70 percent buy the high-priced juice.

- Of all the gasoline stations in Salt Lake, 85 percent are self-serve.

These are highlights from a survey conducted during December by Whitney Leigh Corp., an oil industry research firm based in Tulsa, Olka., that runs annual price and sales volume surveys of gasoline in 44 U.S. cities.

Of the Salt Lake Valley's 377 (according to Whitney's census) stations - both self-serve and attendant-served - 192 were randomly selected, monitored and the results extrapolated for the entire market.

All the data from this and the other markets are fed into Whitney's computers and sold to subscribers.

According to McDonald J. Beavers, Whitney Leigh president, Salt Lake gas prices are "on the low side" nationally. (Figures on gasoline prices are "credit prices" and do not include discounts for cash, a practice found in a few stations.)

Beavers is intrigued with the low usage of premium fuel locally compared with national figures. He speculates there are several reasons:

- A higher ratio of older, pre-1975, cars (the year when installation of catalytic converters to control air pollution mandated unleaded fuel to prolong converter life.)

- "Misfueling" - cheating - by owners of post-1975 cars who alter their fuel fillers to accept the larger, leaded-gas nozzles. (The Environmental Protection Agency says 16 percent of all vehicles are misfueled.)

- A tendency of budget-minded Salt Lakers to buy the cheapest gas they can find even if they believe their car will run better with premium. Ironically, Beavers points out, most car engines are tuned to burn regular unleaded fuel but, nationwide, many motorists insist on premium.

In any event, time is running out on misfuelers and owners of older cars alike, said Beavers. In some eastern markets, many oil companies no longer carry regular leaded gas in their stations. Salt Lake is not far behind.

"Right now," said Beavers, "23.4 percent of Amoco's, (parent company of Rainbo Oil) sales are regular leaded. When that comes down another 10 points they will drop it entirely."

Beavers says Amoco will then bring in a midgrade unleaded to replace regular leaded. "So, what they'll end up with is an 87 octane regular, an 89 octane midgrade and a 93 octane premium, all unleaded."

He predicts that within five years, regular leaded gasoline sales will be down to 5 percent of the total. "You'll have a hard time finding it and you'll have to pay more when you do."

As for diesel, it is already down to 2.2 percent of all fuel sold in Salt Lake. Eventually, said Beavers, it will be found only in interstate truck stops - where it was before the diesel boom of the 1970s brought it into the city.