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.
Fully upgrade a vehicle
Accept and beat a friend's Challenge
Achieve a podium in the Pro final
Pull off an Insane T-Bone in Rampage
Complete a Race-Off, Rampage and Head 2 Head Event
Complete a clean run in a Head 2 Head event
View a Crashback replay
Get the First Blood bonus in a Knock Out event
Wreck your first opponent in a Rampage event
Complete your first Showdown Tour event
Send a Challenge to a friend
Complete a Splitscreen event
Be in 1st place during each elimination period in an Eliminator event
Win a Smash Hunter 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
Complete a Showdown Tour event in each location
Own 15 vehicles
Win a Race-Off event without boosting
Use a full tank of boost in one go without crashing in a Race-Off event
Achieve a podium in the AllStar final
Achieve a podium in the Champion final
Win an Online team event
Collect all 100 Hidden Packages in Joyride
Wreck 20 vehicles in the Lafayette Eulogy
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
Achieve a 5x Wrecking Streak in Rampage
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