stock tires, i.e. the ones they came with...if u got a miata like mine (105000+ miles)...and only changed them @ around 70000, then sure, the tires r pretty f'd already. i knoe it's RWD...i tried to fly around a corner in it by drifting...had to pull the ebrake 'cause if i tried to break the rear end loose w/ throttle input, it wouldn't break. if i kicked the clutch it might just go but car would just spin out instead of doing the nice drift. pulling the ebrake won't help "drive"...it's gonna help the rear break loose.
bad tires would be tires that were driven hard or over 65000 miles. i think...not sure exactly.