Full list of all 49 DiRT Showdown achievements worth 1,000 gamerscore. It takes around 25-30 hours to unlock all of the achievements on Xbox 360.
You won a Speed Skirmish event
You won an Online event as part of a team
You completed all Joyride Missions
You completed 100% of the Yokohama Compound Missions
You collected all 100 Hidden Packages
You completed the Donut King mission in the Yokohama Construction Site area
You completed 50% of the Yokohama Compound Missions
You completed 100% of the Battersea Compound Missions
You completed 50% of the Battersea Compound Missions
You wrecked 20 vehicles in the Lafayette Eulogy
You pulled it together and pulled the opposition apart to achieve a podium in the Showdown Tour
You were in 1st at each Elimination period
You achieved all hit types in Rampage events
You fully upgraded all your vehicles
You own all vehicles
You completed the Showdown Tour and won Gold in each event
You achieved a podium in the Legend final
You completed the Showdown Tour with at least a Silver in each event
You completed the Showdown Tour with at least a Bronze in each event
You own 15 vehicles
You earned a total of $1,000,000
You achieved a 5x Wrecking Streak in Rampage
You reached Fanbase Level 10 Online
You achieved a podium in the Champion final
You completed a Showdown Tour event in each location
You survived for 2 minutes or more in Hard Target
You dominated a friend in Challenge mode
You scored 300,000 points in a Trick Rush event in the Showdown Tour
You achieved a podium in the AllStar final
You Dominated the track on the final lap
You completed a Splitscreen event
You won a Race-Off event without boosting
You came 1st in a Smash Hunter event
You completed a clean Head 2 Head run
You achieved a podium in the Pro final
You got the First Blood bonus
You fully upgraded a vehicle
You completed a Race-Off, Rampage and Head 2 Head event
You sent a Challenge to a friend
You took on and beat a friend's Challenge
You pulled off an Insane T-Bone
You wrecked your first opponent in Rampage
You completed your first Showdown Tour event
You viewed a Crashback replay
You used all your boost in one go and kept out of trouble
Hold the loot for 2 minutes in Smash & Grab
Drop the Flag off 5 times in a round of Transporter
Reach Fanbase Level 30 Online
Reach Fanbase Level 20 Online