Here is the full list of all 48 MXGP - there are The Official Motocross Videogame achievements.
Win a Championship in any class in Career mode
Win a race at Teutschenthal in any offline mode
Complete one valid race on each track in the game in any offline mode
Win a race at Matterley Basin in any offline mode
Get your picture on the cover of "MXGP Mag"
Win a race at Hyvinkää in any offline mode
Sign your first contract in the MX1 class in Career mode
Win a race at Uddevalla in any offline mode
Ride for a total of 300 km (187 miles) in any offline mode
Win a race at Maggiora in any offline mode
Get 40000 fans in Career mode
Win a race at Ernée in any offline mode
Get 10 Holeshots in any offline mode
Win a race at Beto Carrero in any offline mode
Get enough Experience Points to reach MX Level 20
Win a race at Valkenswaard in any offline mode
Get 20000 fans in Career mode
Win a race at Si Racha in any offline mode
Win a race at Losail in any offline mode
Get enough Experience Points to reach MX Level 10
Complete an Online Grand Prix
Win a race at Kegums in any offline mode
Win a race starting from the last position
Win a race at Bastogne in any offline mode
Enter Replay mode for the first time
Select the Pro Physics setting in the Riding Assists section and complete a valid race
Win a race at Lausitzring in any offline mode
Earn enough Experience Points to reach MX Level 5
Access your Social Feeds page in Career mode
Achieve a Team Objective in Career mode
Get pole position and win both races in the same offline Grand Prix
Raise your MX Level high enough to unlock a team
Win a race at Agueda in any offline mode
Raise your MX Level high enough to unlock a picture
Get pole position in an offline Qualifying session
Get the Holeshot in any offline mode
Raise your MX Level high enough to unlock a helmet
Spend a total of 2 minutes airborne from jumps in any offline mode
Sign your first contract in Career mode
Enter the Bike Set-up menu in the Pit
Create your own custom rider
Change your rider's helmet in the Rider Customization section
Get 1000000 fans in Career mode
Compete in valid online races for a total of 60 minutes
Compete in valid online races for a total of 3 hours
Complete an Online Championship
In Online Season mode, get promoted to the MX1 class
In Online Season mode, get promoted to the MX1 Elite class