Complex sentence: Galmer was an American racecar manufacturer that built cars used from 1992 through 1993 in CART competition and the Indianapolis 500, most notably Al Unser Jr. 's winning car in the 1992 Indy 500 in the closest finish in race history.
Split into simpler sentences: Galmer was an American racecar manufacturer that built cars used from 1992 through 1993 in CART competition and the Indianapolis 500. It's most notable accomplishment was Al Unser Jr.'s winning car in the 1992 Indy 500 in the closest finish in race history.