Go to view cars, then choose to walk around in your garage. Go in the back and you will see two games set up, called Geometry Wars 1 and 2.

Get the required amount of Kudos in Career Mode to unlock the secret concept cars.

Cadillac 16 90,000 Kudos
Ford GT90 300,000 Kudos
Ford Mustang GTR 10,000 Kudos
Ford Supercar Concept 120,000 Kudos
Nissan GTR 50,000 Kudos
RUF Supercar Concept 475,000 Kudos
Shelby Cobra 160,000 Kudos
Shelby GR-1 390,000 Kudos
Toyota GT-One 230,000 Kudos

Xbox 360 Achievements  -  Listed below are the 20 achievements and the points given for achieving them.

Unlockables

Arcade Player (15) Play Geometry Wars 1 and 2 in the garage

Bronze Champion (50) Complete all championships at Bronze level

Exotic Car Club (45) Own: McLaren F1 LM; Shelby GT 500; Ford GT-40 MK 1; Corvette ZR1; TVR Cerbera Speed Twelve; RUF CTR \"Yellow Bird\"; Callaway Sledgehammer Twin Turbo

Ferrari Owners Club (45) Own all the Ferrari cars

Gold Champion (75) Complete all championships at Gold level
Gotham Hero (30) Be featured on Gotham TV
Gotham TV Sports Fan (15) Watch Gotham TV 10 times; save a Replay of a race

Lamborghini Owners Club (45) Own all the Lamborghini Cars

Online Professional (60) 50 online races; start last/finish first; 3-event win streak; clean race; win each scenario

Photographer (10) Take and save a picture in each of the different cities
Platinum Champion (100) Complete all championships at Platinum level
Pro Racer Badge (60) Earn a Platinum medal using manual transmission

Race Against The Clock (50) Set a lap time for every circuit in Race Against The Clock mode
Rank 1 (80) Get to Rank 1
Rank 10 (20) Get to Rank 10
Rank 5 (50) Get to Rank 5

Silver Champion (60) Complete all championships at Silver level
Steel Champion (30) Complete all championships at Steel level
Style Racer Badge (50) 25m powerslide; 10x Combo; 25k Online Kudos; 1k Kudos in a race; Kudos for every maneouvre

Track Builder (10) Create and save 10 Custom Routes