IN SPITE OF of all the defects in the bill to raise the gasoline tax, the Senate has a responsibility to pass it. In spite of that gross and disgraceful cave- in to the trucking lobby, the Senate ought to pass the bill -- at last.

Conceded, there's much to detest in this battered bill as it now stands in its final, take-it-or-leave-it version. Have you ever tried to pass a freight train on the highway -- or to keep your own car on the road while it's being passed by one? No? You will shortly have that opportunity. In a series of outrageous concessions to the trucking industry, Congress has attached to this bill a series of provisions permitting much longer and heavier trucks on the road. The truck with two trailers will now make its appearance on the crowded highways of the East Coast. A word of caution: watch out for that second trailer. It tends to swing in a crosswind. It will be a little wider than the truck trailers to which you are accustomed. Congress has already passed, in an appropriations bill, an expansion of the legal width from eight feet to eight and a half. It will be interesting to see what happens to the accident rates.

All of these expansions of weight, width and length -- and the overriding of lower limits set by state law -- were supposed to buy the truckers' acquiescence to an increase in the tax on them. But, as it turns out, that burden will be exceedingly modest. Over the next five years it will ease gently up to $1,900 for the heaviest trucks -- those over 40 tons. That won't begin to pay for the damage to the roads.

There is also the gasohol exemption. The original Senate bill totally exempted motor fuel from the tax if grain alcohol was mixed with the gasoline. The idea was to encourage Americans to pour the world's food supply into their gas tanks, in the hope of raising farmers' prices. If this misguided exemption had remained, it alone would have justified a veto. But as the bill has now emerged from the conference, it would put a tax of four cents a gallon on gasohol. That's a tolerable compromise, if not ideal.

Balanced against all of these shortcomings, there is one central reality. This country is now running out of public revenues for basic public facilities like roads. This bill would raise $5.5 billion a year -- most of it for roads, some of it for transit. This nickel increase falls well short of restoring the original purchasing power of the present four cents when it was first imposed 23 years ago. Gasoline prices have recently been falling, which makes the present moment a good one to raise the tax. A nickel a gallon isn't much. But it can make an important contribution to the maintenance of the country's transportation system.