If you're considering buying winter tires, you want to go *narrower*
not *wider.*

that said, good luck finding anything narrower than a 205 in a 15"
size.  so just stick with the stock size

nate

Winter tires are usually narrower and proportionately taller (higher in
aspect ratio) than an all-season or summer-performance tire for the
same car.

You can probably get away with the same wheels from a fitment
standpoint (tires being made of rubber, each rim width can accommodate
a range of tire widths).  But a lot of people find it convenient to
pick up a cheap set of steel wheels so that (a) they can keep both sets
of tires mounted and (b) they don't trash their nice  "mags" in winter
driving, with its unseen potholes, big hard ice'n'gravel treats,
uncertainty about just where the curb is, etc.

Either way, you also have to have a storage shed, or a tolerant
landlord, or a big trunk, or a girlfriend who watches all those home
improvement shows on cable TV and  thinks it's really cool how you
turned your extra tires and a piece of glass into a post-industrial
coffee table, or something like that.

Much good advice on choosing winter tires (and driving safely in ooky
conditions) may be found on tirerack.com just to take one example.

You can change either the drive or the driven gears of the speedometer
to compensate, but most people probably settle for measuring or
calculating the difference and then just doing eyeball compensation on
the indicated speed.    (It's a good idea to run past one of those
radar sleds or a measured mile once in awhile anyway even if you have
the OEM tire size, just to see if your speedometer is telling the
truth.)

Cheers,
--Joe