The growth of the fleet coincides with the sharp expansion of the city’s work force under Mr. de Blasio, a surge in head count that the mayor has proudly defended as necessary for serving the population of a growing city. There are more than 294,000 full-time employees of New York City and the equivalent of 25,000 more who work part time.

But transportation advocates have expressed concern that the attendant rise in driving, which has outpaced the growth in head count and has not been previously reported, could undercut some of Mr. de Blasio’s signature goals. More driving makes it harder to reduce the number of traffic fatalities, which Mr. de Blasio has vowed to reduce to zero, and to address the pollution that contributes to climate change.

In recent days, Mr. de Blasio has announced his intention to mandate energy efficiency from large buildings across the city, and unveiled new electric vehicle charging stations that will eventually arrive in each of the five boroughs to encourage New Yorkers to buy zero-emission vehicles.

“This is nothing less than an existential threat and every person, every state, every city has to be a part of the solution,” Mr. de Blasio said on Wednesday of climate change.

But the increase in driving runs afoul of his plan for reducing the city’s emissions 80 percent by the year 2050, which calls for New Yorkers to “significantly reduce the number of miles driven.”