If you're looking for manga similar to G, you might like these titles.
In the world of motocross, 16-year-old Satoshi Ichimonji fights his way from the novice grade up to Junior Cross championship.
The series follows Kenji Hatano, a young man who sets out to master conquer the world of kyōtei (hydroplane racing). Over the course of the series he develops a serious rivalry with fellow racer Hiro Doguchi.
Arata, Chinta and Buuyan found an old used car in the dump. They tried to fix it, but it exploded. They decide to try fixing another car, which is an Eunos Roadster. They need 10 years to get it finished and by then they their dream is not so simple anymore...
It follows the adventures of a boy named Yonkuro Hinomaru as he races Mini-4WD cars.
The Spin off takes place in a very different setting, where instead of the Mafia, Racers exist. Racers use go-karts, and during a race anything goes, including any type of interference to prevent your opponent from winning. Tsuna is the descendant of the person know as the greatest racer in history, Primo but is absolutely horrible in the sport himself and has lost 999 races in a row. Tsuna races for Team Vongola, and is known as the disgrace of Nammimori town.
Sylphid is a horse born on a small horse ranch. Even though he is named after the fairie wind he must overcome the handicap of having been born with tendonitis of the leg, and the loss of his mother during his birth. Hayao; the young boy who loves horses, is determined to see Sylphid grow up healthy and become a top racer like his mother once was. However, the odds are stacked against the orphaned foal and the young boy.