I believe that the question is referring to what is called a Pace Car which generally pulls in front of the pack of cars to control the speed and position of the following vehicles.
Usually, whatever the pit road speed that has been set for that race is.
It depends on the track. Since different tracks have different pit road speeds, the pace car will drive about ten mph faster than the pit road speed under caution laps. At the beginning of the race, however, the laps that the cars do before the green flash drops (also called parade laps) are done at pit road speed. This is so the cars can match their rpms with the pit road speed, as NASCAR does not allow any kind of speed monitoring devices in the vehicles.
Because they take at least one lap at the pit road speed, and the cars don't have speedometers, they can verify what the tach says at that speed. Whenever you've got a bunch of cars lined up their speed will vary as cars ahead speed up and slow down. They split the pack so that the cars in the back half of the field get a more accurate reading (the pace car drives at a set speed, so there are fewer cars in front to bunch up and stretch out.
The transponder, by sending a signal to the pit wall, is used to identify individual cars on the track, provide accurate lap timing and overall position in the race.
The pit crew does not build engines. The teams engine department does this. The pit crew services the car during a race by fueling, changing tires and fixing the car so it can return to the race if the car gets damaged.
The fastest pit-stop performed during an official F1 event currently stands at 2.31 seconds set by Vodafone Mclaren Mercedes in the 2012 German Grand Prix at Hockenheim for Jenson Button during his second stop of the race.
Racing motorbikes, with the exception of the Isle of Man TT are only allowed one pit stop and that is to change from wet tyres to slicks if the race is declared a wet race from the start - not including endurance races - if a race is declared to be dry at the start and it rains, the race is stopped until the weather clears up and it is then restarted, on slicks.
There are a variety of special actions that can influence each leg of the Amazing Race, in addition to the road blocks and pit stops there are: Fast Forwards, U-Turns, Switchbacks, Yields, Speed-Bumps, Express Passes
* Pit Road * Pit Lane * Pit Stall
It depends of the track. Short track like Bristol or Martinsville, the speed limit in 45mph because the pit lane in narrow and the cars are closer to each other.
these bikes are often ridden around at drag strips and race tracks in the "pit."