I have a '06 3 with about 6k miles on it... had to replace my right rear tire not too long ago. so at this time, front tires have maybe 50% tread left, left rear has about 70-75% left, and right rear is virtually brand new at this point. just rotated them front to back (new tire is directional, didnt want to pay $180 for a new OEM) so new tire is on front right...
question is, car now pulls to the right when accelerating/braking, not so much when just cruising on a flat road. if i hold the wheel straight, it goes straight. should this be expected since i have two different tread levels on the front? anything to worry about or should i just deal until its time to rotate again... funny thing is, before, my car seemed to drift a tad to the left.

on a side note, got my oil changed at the dealer and overheard a service tech telling a customer she needed new spark plugs... that'd it cost about $230... sounds a little ridiculous to me. what all is involved in a tune up with mazdas? with my toyota before i got the mazda, whenever i had to have work done on it, i'd ask a friend who's a toyota mechanic to help insttead of paying dealer labor charges... i'd just buy him dinner or something and he'd do the work for free, i just had to buy the part. so i'm used to paying like $15 for spark plugs, what's the other $200 involve?

Wait, so you have a different kind of tire on your front right and the stock one is on front left? It's pulling because the tire you bought grips either better or worse than the stock one. It's probably a different size (even if they're both 205/45/17 or whatever the stock size is, different tire models are different sizes) so it's probably tearing up your transmission. You should never have two different models of tires on your drive axle.

Solution= either buy a new tire for the other wheel up front (best option) or just keep the different one on the back until it's time to buy new tires.