According to Official Guinness Records,
The fastest outright speed recorded in a Grand Prix race is 372.6 km/h (231.5 mph) by Juan Pablo Montoya (Colombia) in a McLaren-Mercedes at Monza in the Italian Grand Prix on 4 September 2005. Montoya started the race in pole position, and led from start to finish to win.
For a complete list of 2005 records, please visit 2005 Guinness Records in Italy.