From Lexicanum

The Kult of Speed is an Ork affiliation. Its members are Speed Freeks, Orks obsessed with and addicted to speed. Although most Orks prefer slaughter and explosions, some become addicted to driving really fast. These crazy individuals band together in their own mobs and warbands, full of War Buggies, Warbikes and Wartrukks. Most vehicles are painted red as, according to Orky superstition, "Da red wunz go fasta!" They rely on Mekboyz to build and maintain their vehicles, and sometimes a warband is even led by a Big Mek instead of a Warboss. The obsession with speed can effect any Ork - so it is possible to find Orks from different Clans in a Speed Freek force. Of all the Clans the Evil Sunz have the most Speed Freeks.

Ork Clanz	Bad Moons • Blood Axes • Deathskulls • Evil Sunz • Goffs • Snakebites • Speed Freeks • Feral Orks • Freebooters

Sources

• 'Ere We Go: Orks in Warhammer 40,000, The Kult of Speed
• every published Ork codex