According to the NOW Comics series
Gregory "Speed" Racer and his older brother Rex were both adopted by Pops Racer and Mom Racer (younger brother Spritle was also adopted and seems not to be of blood relation to Speed or Rex).
When Speed was seventeen, Rex died driving off a cliff. The police said that perhaps he was drinking or driving too fast. Speed knew this couldn't be the case (it wasn't, his car was shot at and he was forced off the road). Rex's body was burned beyond recognition and his Army dental records needed to be checked to confirm his identity (obviously a cover-up).
Pops felt as if he'd let his family down now. He and Rex worked together to build the Mach V. Pops had borrowed money from the bank in order to fund the car. Now he would have no choice but to sell the car to pay back the bank. He wasn't as skilled a driver as he used to be and the computer assistance in the car needs special training.
Before Pops could make a move, Speed declared that he could race the Mach V. He said that Rex taught him everything before he died. Slightly angry at the idea of Speed risking his life, Pops went on to point out the dangers of racing and the fact that Speed was simply too young to be allowed to be a professional racer. Again Pops says he feels as if he's failed them since he promised to provide and raise them when he adopted them. Speed assures Pops that he didn't fail them and now he must do his part for the family.
Speed heads to the race track to talk to the race officials about being allowed to race. He is jeered by the other racers who give him a hard time about his age and say that his brother was a drunk. Speed begins fighting with them but is soon out numbered. Before things could progress, the racing official known as The Colonel arrives to put a stop to the violence. He and Speed go to his office to speak.
Speed tells the Colonel and the other officials that he can race the Mach V, win, and pay back the bank. They admire his confidence but point out that the rules state the drivers must be eighteen. Angered, Speed tells them they're just afraid since they own race cars. He says they know he could win and they just want to try to buy the Mach V when they are forced to sell it. Speed leaves still determined to be a racer.
At the qualifying race, Speed jumps the Mach V onto the track when the race begins. He manages to beat all the professional racers and set a new course record. Speed even jumps the Mach V onto the winners stage where the officials are all standing. They are mad at his recklessness but it's the Colonel that points out the skill he's shown. The Colonel tells the other officials that Speed will pack in the spectators to the races and moves to allow the age requirement to be changed.
Speed Racer is on his way the be the best racer there is.
Throughout many dangerous races, the mysterious masked racer known as Racer X frequently appears. There seems to be lots of unexplained accidents whenever Racer X is in a race. The truth is, Racer X is really Speed's older brother Rex. Rex faked his death with the help of a government agency and is trying to fight the evil forces of the world. His identity is a secret to protect his family. He always shows up to make sure that Speed is safe.
Speed Racer was originally based on the Japanese cartoon Mahha GoGoGo in 1967. There were other cartoons in 1993 and 2002. Neither could achieve the success of the original Speed Racer.
In 2008, the Wachowski Brothers directed a live action film.
Also in 2008, Speed Racer: The Next Generation is released by Lionsgate Entertainment. The series takes place years after the original. Speed is not in the picture but has two sons, Speed and X. Both attend an older Spritle at his racing school. The series is set to air on Nickelodeon.