His alternate mode in unknown, but his legs have tank treads, implying a large ground assault vehicle.
Fun Publications Cybertron Fan Club Comic
With Cybertron largely evacuated, Crush Groove followed his friend Jackhammer to Maccadam's Old Oil House, stockpiling a mass of Energon cubes and high-grade oil that had been left behind. The two were sitting pretty until they were approached by Ramjet and Nemesis Prime, who "tested" the two to determine if they would become warrior-slaves to Unicron.
Five seconds later, both Decepticons were dead, and Ramjet and Nemesis Prime moved on.
Crush Groove's name may be a reference to the 1985 film Krush Groove.