www.tiresizecalculator.info will tell you what impact the size difference between a given set of tires would have on odometer and speedometer readings.
Excellent advice, lucaspl88.
One other bit of info you'll need if you buy used steelies is the offset of the wheel, which can be no more than 55mm and no less than 50mm (stock is 52.5mm (crazy)(uhm)(screwy)).
So like I said.....patience for this next question.....so there are tons of P205/55/R16 rims .......but most are identified as being for anything but a Mazda (VW, BMW, Pontiac, KIA, etc).....how do you know what the offset is??? and whether they will fit your car.....I've come across a couple of 1 year old winter packages for a Mazda 3 GT and I'm tempted to just go after them, even though they're a bit more expensive, just because I know they'll fit.....