Please delete this if i am not supposed to post this information. I cannot find anything reguarding not posting this content.
The three achievements i am going to tell you about for this game will give you 75 points if done correctly.
First of all you need to make sure the auto-save is off. If it's on this trick will not work.
The 3 Achievements are as follows.
1) Garage Full (Tuners) 25 Points
2) Garage Full (Exotics) 25 Points
3) Garage Full (Muscles) 25 Points
Now to begin with you should probably wait to do this until you are about 50% into the game, with a little more than $300,000.
Make sure you have unlocked quite a few cars. I didnt keep track but if you have done about or more than 50% of the game there should be enough.
Go to any car dealership and buy all the cars you can of one kind. (tuner, exotic or muscle) To make the first acievement easiest just buy all the cars of the same kind you are currently driving.
Bam! first achievement!
OK next part is to walk over to your 360 and shut it off! OMG! what did you just do! All your progress! OMG!
HAHA! Turn that baby back on and run the game and you'll see that there is no changes to your garage, it will actually be the same as before you got the achievement. All the same money all the same cars.
Now do it again with each car type to unlock all three achievements, after each achievement remember to shut off the console after each acheivement to keep everything in your garage from the begining.
Also a side note: When filling you garage with car type other than what you are driving you will have to go back you your gargage and sell it to make space for the last of whatever one you are trying to do.
If that isnt broken down enough feel free to ask me, and i'll be glad to help out.