Average costs to own and run typical new compact-sized ears in 20 key U.S. cities soared 51 percent in the past two years reaching 47.3 cents per mile in 1981. This is 16.1 cents more than the same driving in 1979, according to a report from Hertz. For the third year, Los Angeles led the list of high-cost driving locales, with a figure of 57.9 cents per mile (CPM). Detroit, on the other hand, was the least expensive of the 20 surveyed cities around the nation, with a cost of 41.9 CPM.

The geographic expense estimate again ranks New York City as the second most expensive spot in the nation in which to own and run an auto, with the typical new compact cost there totaling 56.6 CPM in 1981. San Francisco placed third among the top cities at 56.3 CPM, Miami fourth at 49.7 CPM, Chicago fifth at 48.5 CPM, Seattle sixth with 47.6 CPM, Denver was seventh at 46.6 CPM, Houston placed eighth at 46.53 CPM, St. Louis was ninth at 46.49 CPM and San Diego came in tenth with a cost per mile average of 46.73 cents.

Increases in vehicle purchase prices along with almost prohibitive interest rates on the money to buy the units accounts for most of the overall rise in the figures for both of the past two years, according to the study. The 43.7 CPM major-city average compares with the previously-reported nationwide average of 44.6 CPM, itself almost 50 percent above comparable 1979 costs for new car driving, which stood then at 29.8 cents per mile. The major city costs normally arc higher than the nationwide average because of higher sales taxes and insurance premiums.

Running from 11th to 20th place in the rankings were Boston (46.34), Minneapolis (45.93), Milwaukee (45.85), Pittsburgh (45.71), Cleveland (44.97), District of Columbia (44.63), Dallas (43.58), Cincinnati (42.86), Atlanta (45.85) and Detroit at 41.95 cents per mile, for the typical four year, 10,000 mile-a-year compact car usage.

Los Angeles' percentage rise was lowest of the 20 cities, up only 13.3 percent over 1979 costs, while Boston showed the biggest surge at 23.5 percent. Variations among the cities along with ranking changes usually stern from insurance, interest rate or sales tax rises at different times of the year.

Insurance premiums are a critical factor and can alter estimated average costs by several cents a mile, so Hertz points out that "insurance shopping" can be as important as bargaining over the vehicle purchase price and seeking the lowest interest rates on loans. The expense estimates also include gasoline, oil and other service station items, maintenance, parts and repairs, licenses and fees and depreciation. Figures do not include parking and tolls, since these can vary widely even within the same city, but the report says that in major areas these items could add as much as 5 to 25 cents a mile to the basic totals.

The vehicle purchase price is a key factor in estimating total ownership and operating costs, since it influences the major fixed, or standing, charges of depreciation, insurance, interest rates and sales taxes. These elements add up to about three-quarters of total yearly outlays.

New compact model purchase \ prices in 1981 averaged $7694 for the typically-equipped units most motorists buy, including automatic transmissions, power steering, brakes and air conditioning. These prices ranged from an $8108 average in San Francisco to $7383 in Detroit. Most of the variation is in transportation charges, re­quired equipment and sales taxes. Insurance, license and fees per mile expenses ranged from 18.1 CPM in Los Angeles to 7.5 CPM in Dallas, with the 20-city average 10.6 CPM and the national average coming in at 9.1 cents per mile for these items. Interest costs ranged from 9.6 CPM in Los Angeles to 8.2 CPM in Washington, D.C. Maintenance was 3.9 CPM in San Francisco and just 2.4 CPM in Atlanta. Fuel and other service station items were 10.5 CPM in San Francisco and 8.6 CPM in Detroit.

Government figures estimate 1980 average auto mileage at under 10,000 miles, with a new estimate stating the average unit traveled 8224 miles in 1981, including fleet units. Other studies show that newer cars typically run about 50 percent more miles during the first year of operation than average, and about 50 percent less in their tenth year. In 1979, the costs of running an intermediate vehicle 10,000 miles per year for three years in 20 key cities was 37.98 cents per mile. In 1981, it was 54.06 cents per miles on average, up 42 percent in two years.