Sorry for the stupid question but how do I find the offset number?

And what is it and what does it do?

BTW, I have 215/45/17x7 tires/rims.

Thanks guys!

 

try looking inside the wheel well of your rims.. or the box it came in..

 

The offset is the measurement (in mm) away from the centerline that the mounting surface (behind the lug bolts) are. In more detail, looking at your rim from the top view when the tire is on the ground. The center line parallel top the outside face of the rim is zero offset. from that line, you measure to the mounting surface (inside rim). That would be the offset.

Usually, the rim will have that info inside the rim when you take off from the car, you should see it. Hope that helps!