Forza Horizon 2

As the achievement says you need to have 175 cars in your garage.

Duplicates of cars are okay (I had 167 unique cars and gave up and bought 8 copies of the cheapest to pop the achievement). The fastest way to get this would be to purchase 175 Volkswagen Rabbit GTI's @ $5,000 each. You'd need $875,000 to accomplish that.

- You can get duplicate cars during your free spins which I'd recommend keeping instead of selling off at half price.
- There is a perk you can get (upper left corner of the perk page) which gives a permanent 10% discount on all cars at the Autoshow.

Addendum / updates:
- If you have the January 2015 G Shock DLC, the 3,000 credit Subaru Brat is even cheaper parking space filler.
- If you have Xbox One Smartglass installed on your smartphone/pad/pod: when playing Forza Horizon 2, the Smartglass home screen displays # of cars owned on the FH2 icon. This is updated almost immediately as you buy cars.

Long explanation

175 cars in this game is a lot, if you're going for the 100% in FH2 you're best off leaving this until you've done everything else and received as many cars as you can from the wheelspins, bought as many cars as you needed for the various championships then going to the autoshow to finish it off.

If you want to get this out of the way ASAP though please be aware you will be bored to tears by the 10th car because of the painfully slow, stuttering and numerous menus + loading screens you have to go through to buy a car.

So anyway in my opinion the quickest way to get this done is if you have a large amount of credits which almost everyone will if you've made any progress in the game is to go straight into the Autoshow and buy the Acura RSX which is only a few cars up from when the Autoshow first loads, this removes the time you'd normally spend faffing about having to sort the cars by value then going across to the lowest value car you can buy.

I know it's 9,000 credits which is a few thousand more than the cheapest car but as stated earlier if you've made any progress in the game you won't even bother about the extra few thousand coming out of your credits each time.

You'll also be much happier not having to repeat that cycle of go to Autoshow-sort by value-scroll to cheapest car-wait for it to load- press A to buy- choose to pay with cash/tokens-get forced into the user-created designs- choose 1 and wait for it to load/ go to colours- confirm it to finally get a single car, it's painful enough with the RSX but at least it cuts out the 2nd and 3rd things which saves a ridiculous amount of time over the course of buying however many of them you need.

Short explanation

1. Get as many of the free cars as you can to start off.

2. Play through the game until at least you win the finale for the 1st time to amass some cars + plenty of credits.

3. Go to the Autoshow and buy the Acura RSX for 9,000.

Repeat step 3 as many times as needed.

I found this to be a pretty nice way of going for it when I was finished with all the other achievements I wanted to get in this game. Make sure to have a couple million CR set aside for this one. Here's how:

1) Go to the Hub in Nice Masséna (where you start/edit a championship, go into your garage, etc.)
2) Championship Sign Up > Horizon Hot Hatch > Retro Hot Hatch
3) Select "Purchase Car", scroll to the right until the 2nd to last one comes up called "Volkswagen Rabbit GTI" (for me it was 4,500 CR cause of having that one perk unlocked that lowers the price of cars in general. Apparently it is normally 5,000 CR so still not bad at all) and purchase this car (get whatever design or colour you would like, doesn't matter).
4) On the next screen, after the events are made/reappear, press B.
5) Rinse and repeat steps 2 - 4 over and over until the achievement unlocks.

Happy Hunting!