DiRT 2

For this achievement you need to Drive All 35 of the cars in the game, but not all of the cars are unlocked at the beginning of the game, you will need to have come 1st in all 3 X-Games, every World Tour event (except the Rally World Tour) and the Colin McRae Challenge event to unlock all of the cars.
You can see which cars you haven’t driven by checking your mileage stats for each car in the My Stuff menu.

You will need around $5,000,000 to buy them all, BUT if you don’t want to purchase them all you can play Pro Tour races Online and use the cars for free.
If you want to buy them all and do it in Single Player it’s faster to do so in Free Races, you can just drive a few meters (drive away from the start line) and retire from the race and it will still count.

Here is a list of all 35 cars: Note some cars appear in more than one event type but you only need to drive them once in any type, i shall make the duplicates smaller.

Rally
1995 Subaru Impreza WRX STI (Win the Colin McRae Challenge)
BMW Z4M Coupe Motorsport
Colin McRae R4
Ford Escort MK II (Win all 3 X-Games)
Mitsubishi Eclipse GT
Mitsubishi Lancer Evolution IX
Mitsubishi Lancer Evolution X
Nissan 350Z
Pontiac Solstice GXP
Subaru Impreza STI Group N
Subaru Impreza WRX STI
Subaru Impreza WRX STI [X Games] (Win X-Games America)

Rallycross
^{BMW Z4M Coupe Motorsport
Colin McRae R4}
Colin McRae R4 [X Games] (Win X-Games Europe)
Ford RS200 Evolution (Win the Rally Cross World Tour)
MG Metro GR4 (Win all 3 X-Games)
^{Mitsubishi Eclipse GT
Mitsubishi Lancer Evolution IX
Mitsubishi Lancer Evolution X
Nissan 350Z
Pontiac Solstice GXP
Subaru Impreza STI Group N
Subaru Impreza WRX STI}

Trailblazer
^{BMW Z4M Coupe Motorsport
Colin McRae R4}
Dallenbach Special (Win the Trailblazer World Tour)
^{Mitsubishi Eclipse GT
Mitsubishi Lancer Evolution IX
Mitsubishi Lancer Evolution X}
Mitsubishi Lancer Evolution X [X Games] (Win the X-Games Asia)
^{Nissan 350Z
Pontiac Solstice GXP
Subaru Impreza STI Group N
Subaru Impreza WRX STI}

Stock Baja
Dodge Power Wagon
Honda Ridgeline
Hummer HX
Toyota FJ Cruiser

Raid T1
Bowler Nemesis
Hummer H3
Mitsubishi Pajero Dakar 1993 (Win the Raid World Tour)
Mitsubishi Racing Lancer
VW Race Touareg 2

Trophy Trucks
Chevrolet Silverado CK-1500
Dodge Ram Trophy Truck
Kincaid Ford F-150 Trophy Truck
Toyota Stadium Truck (Win the Landrush World Tour)
West Coast Choppers Stuka TT

Class 1 Buggies
Brian Ickler Buggy
DeJong MXR
Herbst Smithbuilt Buggy
PRC-1 Buggy

This will unlock once back in the trailer

Here's a video showing each car

While this may seem pretty easy, you'll need to play Dirt Tour until you've completed at least the World Tour events and Colin McRae Challenge, as winning them unlocks cars.

If you're not sure which vehicles you have left, you can check each car's total distance driven in the My Stuff section in your trailer.

As well as doing the above you can also race online in the Pro Tour, driving different vehicles in each race as this counts towards the achievement as well, and it saves you having to save up a lot of cash to buy them all.

The events you need to do to unlock the rest of the cars are:

Colin McRae R4 - Beat the X-Games Europe
Dallenbach Special - Beat the Trailblazer World Tour
Ford Escort MKII - Beat all X-Games events
Ford RS200 Evolution - Beat the Rally Cross World Tour
MG Metro 6R4 - Beat all of the X-Games events
Mitsubishi Lancer Evolution X - Beat the X- Games Asia
Mitsubishi Pajero Dakar 1993 - Beat the Raid World Tour
Subaru Impreza WRX STi - Beat the X-Games America
1995 Subaru Impreza WRX STi - Beat the Colin McRae Challenge
Toyota Stadium Truck - Beat the Landrush World Tour