According to Official Guinness Records,
The highest speed on a quad bike (ATV) is 315.74 km/h (196.19 mph) achieved by Terry Wilmeth (USA) onboard his ALSR Rocket Raptor version 6.0, a modified Yamaha 700 Raptor at the Madras Airport, Madras, Oregon, USA, on 15 June 2008.The ATV was augumented with a hybrid rocket thruster. The speed is the average of 2 runs in opposite directions.
For a complete list of 2008 records, please visit 2008 Guinness Records in United States.