Boston — Keeping the family station wagon, the children's mopeds, and the motor boat at the lake filled up with gasoline is getting a lot more expensive. Gasoline and diesel fuel prices across the the United States, which had been dropping late last year, are climbing again.
And lower oil supplies and higher refining or distribution costs are not the whole cause.
Contributing to the steeper at-the-pump prices are motor fuel tax boosts over the past six months.
Particularly hard hit may be Montana motorists, who now shell out an average of $1.247 a gallon for gasoline, 24 cents of which are taxes - 9 cents for federal and 15 cents for state purposes. Besides last April's across-the-nation boost in Uncle Sam's levy, the state, as of July 1, raised its motor fuel taxes by 6 cents.
Montana lawmakers concede that the state increase, coming as it did on the heels of the federal hike, is a particularly hard lump for motorists to swallow. But they maintain they had little real choice, since a substantial portion of roads throughout the state are in deplorable condition.
Of some 5,000 miles of primary state roads, 458 miles ''are in critical shape ,'' demanding early, if not immediate, attention, explains Les Benedict of the Montana Highway Department.
Like their counterparts in many other states, especially in the frost belt, Montana roads are simply worn out and must be reconstructed, Mr. Benedict says. Many were built in the 1930s. Benedict declines to speculate how many of the 787 ,000 miles of highways, secondary roads, and local streets might be involved.
Much of the estimated $32 million in increased revenue that levy is expected to give the state will go for roadbuilding, including Montana's share of matching funds for the Interstate highway system. Some of it, however, is to be used for resurfacing and maintenance projects on which the state must go it alone.
Although the dimensions of the road-construction financing challenge may vary , at least 23 states and the District of Columbia have increased gasoline and diesel fuel taxes within the past nine months.
Last December when the 5-cent hike in the federal gas tax moved through Congress, many close to the state tax scene were forecasting little or no action toward higher state motor fuel levies this year, notes Kathy Yoe of the Highway Users' Federation.
She notes that besides hiking gasoline taxes, lawmakers have provided new revenue for road-related purposes through increases in various road-user fees. At least 20 states have raised various trucker charges and 10 boosted automobile registration levies over the past nine months.
The additional taxes were needed in order to take advantage of the federal highway construction program, and maintenance projects not covered by that matching-fund program, notes Bill Higgins of the American Association of State Highway Transportation Officials.
Of the 18 states that have put higher gas taxes on the books during during 1983, several had previously increased these levies at least once over the past three years.
Instead of imposing a big boost in one step, as was done in Montana, some states have phased them in. ''The idea of adding a few pennies now and more later seems to be becoming increasingly popular,'' Miss Yoe observes. She notes that several states have kept their motor fuel taxes down, but picked up additional money by making gasoline purchases subject to a sales tax or some type of variable levy, tied to cost of living or wholesale prices.
Washington State, for example, upped its levy from 12 cents to 16 last July. Another 2 cents will be added next July. A new Illinois tax-hike measure raised the motor fuels levy 3.5 cents, to 11 cents a gallon, last July. Another penny will be add in July 1984 and 1985.
The 1983 gasoline tax increases are projected to yield the 18 states involved ''slightly more than $1 billion'' in new revenue during the current fiscal year, according to Kenneth Kirkland of the National Conference of State Legislatures. Adding the seven states where rate increases stemmed from actions in past years, the figure may top $1.1 billion, he says.
Besides the 6-cent increase in the Montana, other boosts this year include 5 cents in Minnesota and North Dakota, 4.85 cents in West Virginia, and 4 cents in Colorado and Washington State.
Few, if any, experts suggest that motor fuel levies have reached a peak. But it is generally agreed 1984 will see little action to raise more gas taxes, since it will be an election year.
While the squeeze for roadbuilding dollars is expected to continue and perhaps even increase in the years ahead, construction costs may be coming down. Federal Highway Administration data, covering April through June of this year, show that roadbuilding costs were down 15 percent over the correspondent three-month period in 1982, when they reached an all time high.
Dean L. Morgan, the Highway Users Federation's chief engineer, credits the downward trend to ''efforts to curb inflation, healthy competition, growing efficiency, and responsibility in use of construction dollars.''
''Those who foot the bills through highway-user fees have a stake in seeing to it that these costs don't start skyrocketing again,'' he cautions.