Gas and car sales have fallen, prompting a drop in the two taxes that cover more than half of the state Department of Transportation’s $3.9 billion budget. DOT took in $1.558 billion in gas and diesel fuel taxes and $563 million in taxes on car sales for the fiscal year that ended June 30 — a $66 million drop from the previous year, in a state that is adding 200,000 new residents each year.