You may not notice it in your wallet, but the nation is spending less to own and run its passenger-car fleet, even though there are more cars on the road. American motorists spent a total of $341.8 billion last year to drive to work , shop, vacation, or visit Uncle Ben and Aunt Millie, down 2.5 percent from $350 .7 billion the year before. It was the first dip in peacetime history, according to a study by the Hertz Corporation.
The basic reason for the falloff, the car-rental/leasing firm says, is the increasing age of cars on the road, now put at 7.4 years. As a result, depreciation, usually a big cost factor, was significantly less. The average age of the American car has been on the way up for the past 14 years.
''Vehicle age,'' says Hertz, ''is higher than any time since immediately after World War II, when Detroit was forced to curtail auto output.''
A new vehicle loses an estimated 25 percent of its value the first year, 15 percent the second year, and a declining percentage in succeeding years.
The average cost of driving a car, ignoring its age, fell a record $103, or 3 .6 percent, to $2,744 in 1983. This figures out to about 33 cents a mile for the estimated 8,317 miles the average auto traveled last year.
Besides the drop in depreciation, the cost of gasoline is also falling. In some parts of the country regular leaded fuel is selling for less than $1 a gallon, while regular unleaded fuel is also nearing $1 a gallon or less. The prospects are for even lower prices, in contrast to the usual increase in fuel prices during the summer months.
Interest costs are also off, from 4.44 cents a mile in 1982 to 4.17 cents in '83.
In sharp contrast, however, there was a 10.4 percent rise in maintenance, repairs, and miscellaneous outlays. Because of the aging of the passenger-car fleet, more frequent repairs are required. American motorists spent $96.4 billion on such work last year, vs. $86.4 billion the year before, Hertz reports.
Insurance, licenses, and other fees were also up, representing 6.52 cents a mile.
Motorists spent $80.1 billion on their passenger cars in 1983, a drop-off of estimated per-car fuel consumption climbed just one gallon, to 535, because of improved fuel efficiency of new units, Hertz reports.
Some other facets of the car-spending study:
* US motorists paid an average $9,179 for each of the 8.9 million new cars sold in 1983, up from 7.8 million the year before, when the average cost was $8, 866. This compares with an average $3,404 for a typical 1972 model.
* Comparing ''comparably equipped'' mid-size models, the '72-model price of $ 3,425 compared with $9,770 in 1983, a jump of 185 percent. These similar '83 -model intermediates are 600 pounds lighter and a foot shorter than the comparable cars of 1972.