Here is the full list of all 35 FIM Speedway Grand Prix 15 achievements.
Beat the Stadium record
Complete the Heat
Be the 1st to reach the finishing line
Ride the wrong way
Fall of the bike 20 times
Start before the tape, and don't get the red flag
Beat all stadium records
Ride 7 seconds in a single broadsiding
Ride 10 kilometers in a broadsiding
Ride 1858 laps
Do a wheelie for 3 seconds
Start 3 sec after everyone and win the Heat
Ride at a speed of 120km/h
Win an event
Win every heat in an event
Unlock 1st stadium
Unlock half of the stadiums
Unlock every stadium
Get max points in every event
Complete a Single Event without disqualification
Complete the whole season
Complete a Season on 1st Place at Hard difficulty
Complete Season on 1st Place at Normal difficulty
Complete Season on 1st Place at Easy difficulty
Complete Season on 2nd Place at Hard difficulty
Complete Season on 2nd Place at Normal difficulty
Complete Season on 2nd Place at Easy difficulty
Complete Season on 3rd Place at Hard difficulty
Complete Season on 3rd Place at Normal difficulty
Complete Season on 3rd Place at Easy difficulty
Buy your 1st part in Season mode
Buy all parts in a shop in Season mode
Complete a Season mode without buying a new part
Complete a Season mode without repairing any parts
Earn 100000 money in Season mode