Hey guys so I have a 1996 audi a4 with the factory (I believe 16'' wheels) and one of the tires is bad, which means I have to buy two new ones, I looked at Sears, cheapest I could find with installation was $250 So I decided I could just buy used, so I am looking at Craigslist for tires and rims, I found 17 inch tires and rims (nice rims) with 85% tread left, one problem is is that they are 17 inch and I have 16 inch. Would they fit with no mods? the bolt pattern is the same 5x112. Also they are 225 45 R17... I get the 17 means 17 inches, but what do the other two numbers mean, sorry if this is a noob question, I am just wondering, if this works out it would be great. Thank you!

Also the back two tires have fine tread but are cracked

yes they will fit

The first number is the width in MMs, and the second number is the percentage of the first number in sidewall height.

So the width of the tire is 225 MMs, and the sidewall height is 45 percent of that.

Here's the lowdown.

225 - basically the width of the tire. How wide it is.

45 - the thickness of it. Usually bigger wheels would required thinner/smaller tires. In this case, 17's on Audis are generally paired with 45 and 18's are generally paired with 40 tires. Theres is a calculation involved and we do this to keep the rolling diameter the same if not similar/closest to stock so it won't mess with the speedometer.

Technical term for the "45" is aspect ratio. Hence it means the sidewall is 45% as tall is the tire is wide, or 225mm wide as stated. Throw em on

225/45/17 is the size tire that the b5 s4 came with, so yes, you'll be fine. That's the size I run right now. It's definately not an aggressive tire with a stock offset wheel of 42-43ish. I have 17.5mm spacers on the rear (24.5 final et) and I have to have 2-3 people in my car in order to rub (lowered 1.5 in the rear). I know I'm being a little redundant but they'll fit just fine.

Next time for a simple question like this you could google it. Tire rack has a ton of great articles on tire size/offsets etc. That's where I get most of my info on tires and wheels.

225 = width in MM's
45 = 45% of the width (sidewall height)
17 = fits 17" rim
offset = distance in mm's from the centerline of the wheel to the mounting surface of the hub. Can be positive or negative, usually positive.

In the case of a +42mm offset wheel, there will be 42mm's from the centerline of the wheel to the mounting surface (closer to the face of the wheel). That being said, less of a positive offset will push the wheel out closer to the fender.

If you wonder how well a given tire matches up to the size of a stock tire, google "tire calculator" - there's a very good one that'll come up which is hosted on Miata.net. You can punch in the sizes of the tire you have and the one you want, to see how close in size they are as well as speedometer error. Try to keep it as close as possible. Other factors such as wheel offset can come into play but for tires, that'll give you what you need.