I am lookin at a set of Ion Alloy wheels and the offset is stated at -27mm what the heck does that translate to in American!!
174-5865b part number
http://www.thewheelgroup.com/wheel.asp?b=5&p=416

 

 

ok thanks but that doesnt tell me anything. I know how to measure offset but I figured with a convertor that it would be a 1.06 inch offest that doesnt even sound right.

 

28mms less than half the wheel width back spacing. -27mm=1.06 inches
So on an 8 inch rim that should make the back spacing 4-1.06=2.94 inches
on a 10 inch rim that would make the back spacing 5-1.06=3.94 inches
You are right that it doesn't sound right, but that is what it is suppose to be, see if you can find the wheels somewhere that gives you the back spacing.
This is a stupid way to state it most online wheel sites state it both ways as back spacing in inches and offset in millimeters.

 

The rim you are looking at is 3.44" of backspace.

 

That sounds more reasonable, but how did you come up with that backspacing?

 

that will probably work, may stick out a bit. i can get 4 for $320 plus uncle sam

 

Earlier this year I was looking extensively into wheels, and stumbled on the "truth" of offset backspacing. The centerline of the wheel is measured from edge to edge, then divided in half. The official width of a wheel is bead seal surface to bead seal surface. This equates to about 1 inch difference versus the centerline width. If you measure a 15x8 edge to edge, you will come up with about 9 inches. With this in mind, you can calculate the offset pretty darn close.
Width+1" / 2 = centerline, then compute with offset.
After comparing this with real world numbers, I found it to be quite accurate

8+1 = 9, 9/2 = 4.5" centerline or zero offset. -1.06" (-27mm) = 3.44" backspace.

Rock Toy, I am sure you didn't need THAT much illustration - I put this all in for any other inquiring minds.