Hi -- a few things. First, according to the staff at Columbia University's Social Sciences Library, current city-level statistics for New York City are not easy to find. So in that vein, I am listing some items that I think may help but which I do not expect to have borough-level granularity.

a) The New York DMV is the first place I looked, but unfortunately they don't break out the vehicles by fuel type. See this page http://www.nydmv.state.ny.us/Statistics/regin08.htm -- NYC & some boroughs are listed at the very bottom, but Brooklyn is not.

b) The New York Metropolitan Transp. Council -- http://www.nymtc.org to 'Data & Model' to Regional Transportation Statistics to "2006 Regional Transportation Statistics" (PDF)

c) For "how much fuel purchased?," I would suggest contacting the DOE Statistics folks -- http://www.eia.doe.gov/contactus.html. It is not likely that anyone is tracking (in publicly-available databases, anyway) diesel consumption at the borough level, but if that information is available, the DOE folks should know where. You may end up checking an online directory (Yahoo, newyorkgasprices.com, etc.) to see how many gas stations are listed in Brooklyn and then performing educated guesswork.

I hope that at least some of this is useful. If not, you might find it good to contact the government documents department at Columbia -- http://tinyurl.com/cqhhkh -- I am not in any way affiliated with them. It's just that for data, govdocs people are usually the way to go.

Cheers!

Supplementing the above-noted sources, check out http://www.eia.doe.gov/emeu/states/_seds_updates.html and related pages for state-level statistics.