When Dr. Christopher Davis bought his diesel Peugot last month, he hoped it would provide him some protection against soaring gasoline prices and spot shortages.

"I figure that if they ever went to gas rationing, they won't ration diesel because the economy needs it (for the trucking industry)," he said, "Prices is a factor, but availability is the biggest factor."

But now Dr. Davis finds himself driving from station to station looking for cheap diesel fuel, and he has been shocked to find its price has soared, catching up with the price of regular gasoline.

Not only is diesel fuel expensive, but it is hard to find because few stations carry it and supplies are tight. About 35 stations in the Washington are sell diesel fuel.

Diesel fuel is selling for an average of 71.6 cents a gallon-2.3 cents more than the average price for self-serve regular gasoline-according to the latest American Automobile Association figures for the Washington area.

"There used to be a 5 or 6 cent difference between diesel and regular gasoline (with diesel cheaper)," said Philips 66 regional manager William E Effertz, "so the gap has closed dramatically over the past two years. [Increase] demand is the reason."

One local diesel supplier, Steuart Petroleum Co., has raised the "tank wagon" price it charges local Chevron stations that it supplies with diesel fuel to 69.2 cents a gallon, a 17.5 percent increase since the beginning of the year.

Not only are prices high, but federal energy experts say the nation's supplies of diesel fuel are even tighter than gasoline supplies now because the planting season arrived a month early in the south and because of an exceptionally cold winter.

Farmers use diesel fuel in tractors and other farm equipment , and the cold winter ate up stocks of home heating oil, which is the same petroleum product as diesel fuel.

Charles McCrea of the Department of Energy's fuels branch, said many people call him to ask if they should buy diesel cars.

"I tell them diesel and gasoline come out of the same barrel of oil, so if there's going to be a shortage there will be a shortage of both." said McCrea. Likewise, he said, prices for diesel and gasoline will tend to rise together.

All this may be unwelcome news for the hundreds of people in the Washington area who have rushed to buy diesel Volkswagons, Peugots, Mercedes and other diesel automobiles.

Despite a much higher yield of miles per gallon, "the higher price of a diesel car makes it unlikely that fuel savings will pay back that initial investment unless you drive it 100,000 miles," said the AAA's Bill Toohey Jr., quoting from an AAA engineering department report.

Prices for diesel vaired widely at stations checked yesterday by a reporter.

Chevron dealer Bill Burton was selling diesel for 79.9 cents a gallon at his station at the busy intersection of Bradley Boulevard and Arlington Road in Bethesda.

Burton said he has a "reasonable" profit margin of 10.7 cents a gallon on diesel. He complained that his sales have not increased dramatically on this product because the price he must pay Steuart and pass on to cutomers has gone "so damn high."

Burton recently raised his diesel price to 84.9 for fear the federal government would order a freeze on profit margins the way it has for gasoline, but he had to lower the price again when sales dropped as a result of the increase.

While federal price controls limit what individual dealers can charge for a gallon of gasoline, there are no such controls on diesel fuel.

Diesel was only 66.9 cents a gallon yesterday at Amoco Self Service at the corner of 13th and N streets NW.

"Sales are picking up quite a bit, and I think right now we're pumping close to 15,000 gallons (of diesel) a month," said Michael Greenberg, a partner in a group that runs the station.

Greenberg said his high-volume, gas-and-go station has low profit margins- he makes 5.8 cents a gallon on the diesel-but is still profitable.

Greenberg's diesel tank wagon price from Amocco is 61.1 cents-8.1 cents a gallon less than the 69.2 tank wagon price that Steuart charges Burton's Chevron.

Steuart's manager of gasoline and lubricants, Peter Horrigan, said that such differences in tank wagon prices arise from differences in the complex national and international contracts under which companies like Stewart and Amoco obtain their supplies. CAPTION: Picture 1, no caption, By Joe Heiberger-The Washington Post;Picture 2, Dr. Christopher Davis has had to search for stations selling cheap diesel fuel. By Joe Heiberger-The Washington Post