Grand Theft Auto IV Guide - Brucie's Races
After the “No.1” mission, you can call Brucie to participate in street races taking place around the city. There are a total of 9 races, and each consisting of 3 or 5 laps around a different part of the city. There are three tracks in Dukes/Broker, three in Algonquin, and three in Alderney, so once Brucie tells you that there aren't any races going on, move on to one of the other islands and call him from there.
There really isn't much to these races; get a fast car, meet the others at the start point, follow the waypoints around the track, and be ahead of the pack when the final waypoint is reached. A Comet, Infernus, Turismo, Banshee, or Super GT is best suited for these races. You can use motorcycles too, but they aren't recommended. The quality of your opponents' cars will improve as you win more races, but either way, once you get a good lead you should be able to keep it the whole way through as long as you keep your eyes on the road. You're not trying to break any records here, so once you're in first, just concentrate on avoiding crashes—which can set you back quite a bit—rather than reaching the finish line blazingly fast.
Note: The races can be replayed; just call Brucie a couple of times after he tells you that there aren't any more races, and he will set you up with one of the previous race. Though winning yields little reward in terms of money, replaying these races is a must to unlock the “Genetically Superior” achievement on the Xbox 360 version for which a total of 20 races must be won.