According to the rule book the "Drive axle must be strictly stock for make and model of car. Must not be locked." That's fine with us, since our Mustang came with a 7.5-inch rear, stuffed with a 3.73:1 ring-and-pinion. Depending on your track, this gearing may not be ideal but we hope it at least gets us going, so we don't have to spend the money twice (or three times!) to get the gearing right. The slightly moist cover isn't a great sign, but hopefully the internals are still in good shape.