I need some help. I'm looking at some rims on craigslist. The guy says they are -12 offset. What would the backspace be? I know the differences but not how to convert haha

 

gotta know how wide the rims are.
-12 is how many millimeters the mounting surface is off center. a minus offset will make the tire stick out further than a positive offset.

so if it's an 8 inch wide rim, 4 would be center. then 12mm is about half an inch, so you would be in between 3.5 and 3.75 backspacing.

if it's a 10 inch rim, 5 is center, so it would be like 4.5 backspacing...

there is 25.4mm to an inch

 

What are the rims off of might be easier to figure out if they fit or not lol.

 

Haha I know they'll fit. Just didn't know how to convert. I do now! Thanks guys!

 

cut a piece of wood to match the diameter of the wheel. Bring a measuring tape and measure from the wheel mounting surface to the piece of wood


Backspace

 

I agree with caged's response. Backspace is not relevant to width. With offset you have to know the width to determine backspacing. Seems confusing, but it is not. They use backspacing for clearance specs mainly. Offset is an easier number to relate to the wheels centerline. So you know if the wheel is more under the vehicle or pushed away. Lower offset pushes the wheel out.
You probably know most of those things. Hopefully that will help the next guy searching for backspacing.