After do all the Dirt Tour events for Monte Carlo to unlock
La Grande Victoire in DiRT 3 (GFWL) you will have driven around 47.3 miles.
Mileage counts across all games modes, online, single player and dirt tour but Monte Carlo is only available for
Rally events.
The achievement pops mid race upon driving your 100th mile (161st km) so if you wished you could set your car up on a hairpin and let the car continuously drive off the edge and reset, or you could do the remaining miles in Jam sessions (Whilst trying for
French Connection in DiRT 3 (GFWL)) or you could redo all the Dirt Tour events, its personal preference.