The Virginia House of Delegates voted today for a gasoline tax increase that, if approved by the Senate before Saturday's legislative adjournment, would add 1.6 cents to the cost of a gallon of gasoline.

The proposal, offered by Del. Warren G. Stambaugh (D-Arlington) at the behest of Gov. Gerald L. Baliles, would generate $47.5 million in the fiscal year beginning July 1 that is earmarked to speed up planning for critically needed highway projects.

Also today, House and Senate conferees signed a budget compromise that adds nearly $51 million to the $18.6 billion biennial budget that had been proposed by former governor Charles S. Robb and amended by Baliles.

The budget conferees, headed by Senate Finance Committee Chairman Edward E. Willey (D-Richmond) and Del. Dorothy S. McDiarmid (D-Fairfax), chairman of the House Appropriations Committee, got more than half the additional money as a result of a bookkeeping change that makes available $28 million worth of Richmond, Fredericksburg & Potomac Railroad stock owned by the state.

The biggest chunk of the additional money will go to higher education institutions, including $25 million for computer and high-technology equipment. The conferees increased faculty salaries at state institutions from 9.5 to 12.5 percent, including 11 percent at George Mason University in Fairfax, but they gave the biggest pay raise to future state legislators, whose salary will jump from the present $11,000 to $18,000 after the next election.

The 59-to-36 vote on the gas tax came over the opposition of 25 of the 31 House Republicans and without the support of any of the eight GOP delegates from Northern Virginia.

"Fairfax Republicans love to shoot off their mouths about highway improvements," Stambaugh said, "but when it came time to act, they wanted to delay." He said defeat of his measure would delay for a year the work on the Springfield bypass and widening of Rte. 28.

Del. Vincent F. Callahan Jr. (R-Fairfax) said Republicans are "fully committed" to funding transportation projects, but said action should be taken at a special September session of the legislature, called by the governor, which will deal exclusively with transportation.

"A 1-cent tax increase won't make a difference," Callahan said. He called the "11th-hour appeal" for a tax increase "embarrassing," and criticized Democrats for "caving in to arm twisting" by the governor and his staff.

Stambaugh called the House action "the first step in our commitment" to tackling the state's transportation problems. The special session, he said, is likely to approve "massive financing" of road, rail, air and water projects, but without preliminary planning and engineering studies, "we can't lay a foot of concrete."

Approving his interim measure, he said, would allow the highway department to begin the three- to four-month process of interviewing additional employes so they will be ready to go to work when the tax increase takes effect July 1.

The House-passed measure is a compromise version of a more ambitious funding proposal backed by Willey that has been approved overwhelmingly by the Senate.

After a meeting with Baliles Wednesday, Willey agreed to support the House version, but he indicated today that his support may be lukewarm when the compromise version comes back to the Senate for a vote.

"The Senate can make up its own mind," sulked Willey. "I'm not going to make any speech" in favor of it.

The majority of Democrats rejected several Republican attempts to gut the bill, but went along with a sunset provision, proposed by Del. John Watkins (R-Chesterfield) that ends the tax increase in 1988 unless it is passed again.

A section of the bill that the GOP particularly objected to gives the governor veto power over projects selected for accelerated planning. Del. James H. Dillard II (R-Fairfax) said the veto provision "could lead to all kinds of pork-barreling, recriminations and blackmail."

The bill adds one penny to the present 11-cents-a-gallon gasoline tax, but has the effect of adding another six-tenths of a cent per gallon at the pump by converting what is now a 3 percent franchise tax on oil companies to a straight 3 cents a gallon. It also retains a subsidy for Virginia-produced gasohol but limits the subsidy to the current level.