I just changed over my fiance's '04 Mazda 3 to winter rims/tires and I noticed that both rear tires are "stepped" on the inner tread. It's like every other tread block is worn down. I googled it and I think the correct term for this is "scalloping". What I gathered from my googling is that weak shocks and/or worn suspension components could be the culprits, BUT I wasn't sure if that's still the case when the 'scalloping' is only on the inner side of the tread. Does that implicate anything else?
FYI the car has about 120,000 km (75,000 miles) on it and is a highway commuter which does a 200km (125 mile) round trip from 2 to 5 times a week.
Any suggestions/ideas would be greatly apprecaited. I don't want my fiance putting a ton of k's on these brand new winter tires and messing them up too.

As an update - I brought the car to a local Mufflerman and had the aligment checked out. Everything is within factory spec. After this I decided to change the rear shocks, which I did the other night. Once removed, I checked the old shocks and both seem to have the exact same compression/decompression characteristics as the new ones (when compressing by hand and allowing to decompress on their own). So now I'm at a bit of a loss ... where do I go from here?

 

The camber on these cars is set up, like alot of newer sport type cars, to angle in toward the vehicle. This will cause the inner of the tire to wear differently from the outer edge.

As for the "scalloping" I'm not exactly sure.

 

Yah, I can tell just from looking at the rear end of the car that the camber is set so the tires would wear more on the inner edge. My concern is the scalloping though. If it was even wear, I wouldn't have any concerns.

 

I've noticed the same thing on both my 3 and my wife's 6. I believe it's due to the negative camber set in the rear suspension to aid in handling. Regular rotation of the tires helps. I got ~40,000 miles out of tires before replacing. (Treadwear rating 300 tires)

 

Your tires have not been balanced and rotated properly. Scalloping can be from worn shocks or bad balancing. It's basically the tire bouncing down the road and hitting your more severely worn tread patches harder on the road than the others. Camber will not do anything to it. And once scalloping has started, you can't reverse it.

 

I'm tellin' ya, the tires/wheels are in balance. As for rotation, I can honestly admit that they haven't been rotated in about 25,000 miles ... ooops. Because of the lack of rotation I could definitely see the majority of the wear being on the inside of the tread, but the scalloping has got to be caused by something else. Since the wheels and tires are in balance and the shocks are fine, I just want to make sure it's not something else ... before the new winter tires get mangled.

 

Although the alignment was checked, you could have a worn suspension component that only manifests a misalignment under the load of driving. Perhaps you could check your suspension bushings for wear... Just a thought.