Here is the full list of all 49 DiRT Showdown achievements. It takes around 80-100 hours to unlock all of the achievements on Windows.
Win a Race-Off event without boosting
Be in 1st place during each elimination period in an Eliminator event
Achieve a 5x Wrecking Streak in Rampage
Win a Smash Hunter event
Own 15 vehicles
Achieve a podium in the Champion final
Complete a Showdown Tour event in each location
Achieve a podium in the AllStar final
Pull off an Insane T-Bone in Rampage
Send a Challenge to a friend
Fully upgrade a vehicle
Achieve a podium in the Pro final
Get the First Blood bonus in a Knock Out event
View a Crashback replay
Complete a Race-Off, Rampage and Head 2 Head Event
Complete a clean run in a Head 2 Head event
Wreck your first opponent in a Rampage event
Complete your first Showdown Tour event
Complete a Splitscreen event
Dominate all sectors during the final lap in a round of a Domination event
Be in last place and then earn a podium during double points in a Rampage event in the Showdown Tour
Use a full tank of boost in one go without crashing in a Race-Off event
Win an Online team event
Collect all 100 Hidden Packages in Joyride
Wreck 20 vehicles in the Lafayette Eulogy
Accept and beat a friend's Challenge
Complete the Showdown Tour with at least a Bronze in every event
Own every vehicle in the game
Complete 50% of the Yokohama Compound Missions
Achieve all hit types in Rampage
Complete 50% of the Battersea Compound Missions
Hold the loot for 2 minutes in Smash & Grab
Score 300,000 points in a Trick Rush event in the Showdown Tour
Complete the Donut King Mission in the Yokohama Construction Site area
Earn a total of $1,000,000
Drop the Flag off 5 times in a round of Transporter
Complete the Showdown Tour with at least a Silver in every event
Reach Fanbase Level 30 Online
Survive for at least 2 minutes in a Hard Target event
Reach Fanbase Level 10 Online
Win a Speed Skirmish event
Complete 100% of the Battersea Compound Missions
Complete 100% of the Yokohama Compound Missions
Achieve a podium in the Legend final
Dominate a friend in Challenge mode
Complete all Joyride Missions in both locations
Fully upgrade all vehicles
Reach Fanbase Level 20 Online
Complete the Showdown Tour and win Gold in every event