may i take a guess you have some cars that u redeemed via dlc maybe the gamestop anniversary cars?? if you do those dont count toward your garage count, i had to hit like 74 or 75 before i got the trophy for 50 cars because i have the anniversary cars, dont remember if the mercedes vision cars counted or not, but dont worry you'll get there. i'm a handful (like 8 or 9) away from 100 cars trophy, but it'll have to wait after work + my nephews school performance.
