Many top enviro groups made a last-minute appeal to Pres. Clinton yesterday to veto a transportation spending bill with a provision that would prevent the federal government from even studying the possibility of raising fuel-efficiency standards for new cars and light trucks. But White House officials said they expect Clinton to sign the bill. Enviros had hoped Vice Pres. Al Gore could push Clinton on the issue, but most don’t blame Gore for the setback. The popularity of gas-guzzling sport utility vehicles has pushed the nation’s average automobile fuel economy to its lowest level in 20 years, 23.8 miles per gallon, according to a new EPA report.