Virginia Gov. Gerald L. Baliles today won legislation that will increase gasoline taxes and give him unprecedented authority over some highway projects, but he appeared to be losing the mandatory seat belt bill he wanted.

In a surprise move late today, the state Senate derailed, and possibly killed, a bill to require use of seat belts.

The vote came on a 19-to-18 parliamentary maneuver in which Lt. Gov. L. Douglas Wilder, an opponent of mandatory seat belt use, cast a tie-breaking vote to reject a House-Senate compromise. Only moments earlier the Senate had approved the measure 22 to 17.

Seat belt measure opponents, led by Sen. Richard L. Saslaw (D-Fairfax), moved to reconsider that vote. Saslaw said the bill was "an absolute joke" because it could not be enforced.

"The blood of 200 people will be on Saslaw's hands next year," muttered Sen. Charles L. Waddell (D-Loudoun), chairman of Senate Transportation Committee and a supporter of the bill. Proponents, who have said the measure would save that many lives each year, are expected to make a final attempt to resurrect it Saturday, the last day of the 1986 session.

The 33-to-6 Senate vote on the gasoline tax resolved one of the last major issues in the 60-day session, the first for Baliles as governor. Legislators are expected to adopt the $18.6 million, two-year budget on Saturday. Specific terms of the budget have been agreed to by conferees from the two chambers.

The revised gasoline tax plan, introduced for Baliles by Del. Warren G. Stambaugh (D-Arlington), has the effect of raising the price of gasoline 1.6 cents a gallon at the pump, beginning July 1. Proceeds for the first two years will go to a special fund for accelerating highway planning.

The state highway commission will decide which projects will benefit from the added money, but the governor was granted unusual veto power over the selections.

Technically, the gasoline tax bill increases the state tax 4 cents a gallon, from 11 to 15 cents, but it works out to a 1.6-cent increase at the pump. While it adds a penny to the basic rate, it replaces a 3 percent tax on the wholesale cost of gasoline with a straight tax of 3 cents a gallon, which translates to an increase of 0.6 cent at the pump.

Two Northern Virginia Republicans, Wiley F. Mitchell of Alexandria and John W. Russell of Fairfax, were among the senators who voted against it.

The legislators give final approval today to a number of significant bills in addition to the gasoline tax measure. Sent to Baliles for his signature were proposals designed to boost the use of Virginia coal by state utilities, to require banks to warn credit card holders of interest charges, to place a ceiling on interest charged by second-mortgage lenders and to restrict when sen- ators can vote by proxy.

The latter two measures are related to an action last year that resulted in the indictment of Sen. Peter K. Babalas (D-Norfolk) on a misdemeanor bribery charge. Babalas, who excused himself when today's votes were taken, is charged with accepting $61,000 in attorney fees from a mortgage lender and then casting his vote and a proxy for Sen. Edward D. Willey (D-Richmond) that killed in committee a bill that would have imposed the interest ceiling.

The Senate's surprise action on the seat belt bill jolted what had been a low-key but determined march toward a scheduled adjournment Saturday as it dealt with dozens of bills. Opponents of the measure ridiculed the bill's provision that motorists could be fined $25 for not wearing seat belts only if police had stopped them for another apparent violation. Sen. Wiley F. Mitchell (R-Alexandria) also opposed the bill, saying that it had been rendered "a toothless tiger" earlier in the session and was "not even in the range of a mild pussycat."

Sen. Joseph V. Gartlan Jr. (D-Fairfax), who backed the bill, scolded the senators for their "chortling derision." Gartlan said the bill "will save lives. If that's worth laughing about, laugh."

Northern Virginians voting for accepting the seat belt bill were Gartlan, Waddell, Charles J. Colgan (D-Prince William), Edward M. Holland (D-Arlington) and Clive L. DuVal II (D-Fairfax). Opposed were Saslaw, Mitchell and John H. Chichester (R-Stafford).

Sen. J.W. Russell, who initially voted in favor of the seat belt measure today, did not vote moments later when the issue was decided by the crucial tie vote of Wilder, who can vote only to break ties.