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

Manual Ability (20 points): Win a race using Manual transmission.
Sim Master (20 points): Win a race at Hard or Champion difficulty using Simulation riding model.
Who Needs the Front Wheel? (10 points): Perform at least 10 seconds of wheelies and finish the race.
Unicycle Champion (25 points): Win a race performing at least 30 seconds of wheelies.
Scriptwriters' Strike Is Over (15 points): Complete all Scenario challenges.
Italians Do It Better (10 points): Win one race in MotoGP class using one of the Italian riders.
My Head Turns (10 points): Perform one 360 degree burnout and then win the race in any game mode.
I'm Back Home! (5 points): Win a race in Laguna Seca with Hayden in any game mode.
Desert Rock (5 points): Win a race with Stoner in Losail in any game mode.
Drift King (10 points): Win a race performing at least 10 seconds of powerskid.
Singing in the Rain (20 points): Win a race under the rain.
Maiden Victory (15 points): Win your first race ever, in any game mode, with any setting.
Riding Degree (10 points): Win at Assen in a 125cc race.
Small but Proud (10 points): Win San Marino GP using De Angelis or Poggiali.
Finnish Ice Racing Champion (20 points): Win a race using Mika Kallio and performing at least 20 seconds of powerskid.
Dominator (25 points): Win a race with a 10 second gap or more from second place.
I'm Here to Stay (10 points): Win the first race in an official MotoGP Championship mode with Toseland.
Dani, Brake Carefully! (10 points): Win a wet race at Sachsenring using Pedrosa with at least 10 second gap from second place.
Technical Rider (25 points): Win at Mugello, Catalunya, Donington, Assen and Phillip Island in the same Career Championship.
Like the Real Thing (20 points): Win a race at Champion difficulty using Simulation riding model and manual transmission.
Test Rider (10 points): Complete 20 laps in Time Attack mode with the same rider.
Loyalty (10 points): Ride for at least 200 km with the same team.
Rossi Fan Club (5 points): Win 10 Quick Races with Valentino Rossi.
Dedicated to our friends (5 points): Win a race with Polaris World Aprilia Team.
High Noon (10 points): Complete all 1vs1 Challenges.
Photo Finish (10 points): Win a race with a second or less gap from second place.
Here Comes a New Challenger! (5 points): Complete a Challenge.
Am I Racing Alone? (20 points): Win a race with a 30" gap from the second at Medium difficulty or more.
Learning to Test (5 points): Complete a lap in Time Attack mode.
Teddy Bear (20 points): Join the Grizzly Gas Kiefer Racing Team in Career mode.
World Champion (40 points): Win an official Championship in any class.
Shinobi-do (10 points): Win an official Championship riding a Kawasaki.
125cc Champion (30 points): Win the 125cc Championship in Career mode.
250cc Champion (45 points): Win the 250cc Championship in Career mode.
On Top of the World (65 points): Win the MotoGP championship in Career mode.
Retirement (30 points): Finish a Career.
125cc Hero (20 points): Win the 125cc Championship in Career mode at Champion difficulty.
250cc Hero (30 points): Win the 250cc championship in Career mode at Champion difficulty.
MotoGP Hero (50 points): Win the MotoGP championship in Career mode at Champion difficulty.
Legend (70 points): Win a Championship at Champion difficulty using Simulation riding model and Manual transmission.
We Like Mocaccino (20 points): Join the Emmi Caffe Latte 250cc Team in Career mode.
A Step Forward (25 points): Choose a 250cc team in Career mode.
Finally On Top (30 points): Choose a MotoGP team in Career mode.
Globetrotter (30 points): Win all non-european GPs in the same MotoGP championship.
Nothing More Can Challenge Me (50 points): Complete all Challenges.
Street Fighter (15 points): Win an official championship riding a Honda.
King of Spain (30 points): Win Spain GP, Catalunya GP and Valencia GP in the same Championship.

Additionally, there are three secret achievements, all of which are currently unknown:

Secret Achievement (5 points): Unknown.
Secret Achievement (5 points): Unknown.
Secret Achievement (5 points): Unknown.