Warrior buggy - Enter "18041851" as a code to unlock the Warrior buggy.

Mudhog Lightning Bolt livery - Enter "17121973" as a code to unlock the Mudhog Lightning Bolt livery.

Sludgeray Butterfly livery - Enter "18021974" as a code to unlock the Sludgeray Butterfly livery.

"Camo" helmet - Enter "48992519" as a code to unlock the "Camo" helmet.

"Road Addict" jacket - Enter "20061977" as a code to unlock the "Road Addict" jacket.

"Speed Angel" shorts and boots - Enter "91031985" as a code to unlock the "Speed Angel" shorts and boots.

Easier races - Immediately after a challenge begins, go to the menu, and switch to a vehicle that is better suited to winning instead of the default vehicle type. Note: You can only do this one time. If you restart you will be stuck with the new vehicle. This can also be done with Career mode races.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Completion obsession syndrome (40 points): Now, go breathe some fresh air!
Hello, world! (15 points): In free ride, go from Offshore Shack Camp to Tsunami Reef Camp without resorting to the chopper.
Born under a lucky star (15 points): Win your first Star in career mode.
Open your eyes (15 points): Find and contemplate a Vista Point.
Here comes a new challenger! (15 points): Win a challenge.
DIY (15 points): Create a race using the race editor.
All your camps are belong to us (30 points): Unlock all 19 camps for great justice!
Humble beginnings (15 points): Win all three Stars on 'Riders on the Storm'.
Path to glory (15 points): Win all three Stars on 'Twisted Minds'.
Under the limelight (15 points): Win all three Stars on 'All's Well That Ends Well'.
One of each, please! (15 points): Own at least one vehicle of each type.
Hobbyist collector (15 points): Own 10 vehicles.
Regular collector (15 points): Own 40 vehicles.
Obsessional collector (30 points): Own all 74 vehicles.
Hobbyist sightseer (15 points): Find and contemplate 10 Vista Points.
Regular sightseer (15 points): Find and contemplate 50 Vista Points.
Obsessional sightseer (30 points): Find and contemplate all 95 Vista Points.
Hobbyist stargazer (15 points): Win 40 Stars in career mode.
Regular stargazer (30 points): Win 120 Stars in career mode.
Obsessional stargazer (90 points): Win all 216 Stars in career mode.
Hobbyist challenger (15 points): Win 30 challenges.
Regular challenger (30 points): Win 100 challenges.
Obsessional challenger (90 points): Win all 190 challenges.
Hobbyist painter (15 points): Collect 50 vehicle liveries.
Regular painter (15 points): Collect 100 vehicle liveries.
Obsessional painter (30 points): Collect 148 vehicle liveries.
Gear lover (15 points): Own 24 pieces of gear for the driver in career mode.
Gear buff (15 points): Own 48 pieces of gear for the driver in career mode.
Gear freak (30 points): Own 72 pieces of gear for the driver in career mode.
Oil strike (15 points): Collect your first oil drum.
Oil rig (15 points): Collect 100 oil drums on the field.
Black gold tycoon (15 points): Collect 500 oil drums on the field.
A stroll in the park (15 points): Drive a cumulative total of 62 mi (100 km) in free ride mode.
Safari adept (15 points): Drive a cumulative total of 310 mi (500 km) in free ride mode.
Globetrotter (30 points): Drive a cumulative total of 620 mi (1000 km) in free ride mode.
Rocket science (15 points): Travel 80 meters in a single jump.
'Tis but a scratch! (30 points): Finish a career race ranked 1st without wrecking your ride.
Blind man's buff (15 points): Finish a career race ranked 1st without GPS.
Natural selection (15 points): Finish 30 online Career races ranked 1st.
Underground brotherhood (15 points): Play and win 30 online custom races you haven't created.
A prophet in his own country (15 points): Finish ranked 1st on 10 online custom races you HAVE created.
Wriggling about (15 points): Perform 200 tricks (bikes and quads).

Additionally, there are four secret achievements:

Newton was right, after all (15 points): Fall 150 meters.
Squadala, we're off! (15 points): Use heliports 100 times. Faster than a flying carpet, huh?
Sand to Stone (15 points): In free ride, go from Tsunami Reef Camp to Redrock Bluffs Camp without resorting to the chopper.
Salt over Snow (15 points): In free ride, go from White Flats Camp to Pinwheels Ridge Camp without resorting to the chopper.