tires

hi!
6 month ago i`ve got a 2000 bmw 323i.....tires on it seemed to be new.....but after like a few thousand km`s......i noticed that the inner sides of the rear tires have worn out(alot) a guy in the tire store said that under inflation might be the reason.....so i`ve got brand new Fuzion ZRI......but after like 3k i`ve noticed some kind of cracks(on the insides again),but this time tires were not underinflated for sure......
does anyone know what that might be???
p.s: it`s a rear wheel drive,and no I dont spin tires.....
thatnx guys

Most BMW's I see have a lot of negative camber. The top of the wheel is tilted inward towards the car. This increases grip in the turns. I think your car may have a little too much negative camber. You will need to get it adjusted by using a camber kit.