Offset 101
An Introductory Guide to Offsets
by jd posted 2008/-0/7-

Too much positive offset will make you hit your coilovers or chassis, the more negative offset you have will make your wheel stick out. Take note that the width of the rim plays a major roll on your offset. The bigger the rim the less room you have to play with offset sizes.

Conversion: 1 INCH = 25.4mm

example: 17x10 rim is 254mm wide (0 offset for that gives you 127mm sticking out and 127mm going in)
case point and said.
a 10" wide wheel is not 10" physically.

roughly 11" yes.

I measured a 9" wheel, hair under 10".

We all know americans seldom use mm.
so heres a inch table:

http://www.1010tires.com/WheelOffsetCalculator.asp

Thanks for the responses. Again let me say that this is a VERY VERY generalized overview of learning offsets. It doesn't go into detail about wheel lip and flushness. The lip of a wheel takes one more extra step to figure out. Notice some of the pics that were posted above? The spokes (face) of the wheel has a certain size also. You need to take that into calculation to figure out flushness....

Next thing we could do is a chart on what clears coilovers and what doesn't.

Width---------------------Offset

6.5 Inch------------------ ????
7 Inch-------------------- ????
7.5 Inch------------------ ????
8 Inch-------------------- ????
8.5 Inch------------------ ????
9 Inch-------------------- ????
9.5 Inch------------------ ????
10 Inch------------------- ????
10.5 Inch----------------- ????

^^ If we can get something like that filled in, that would be the best guide to buying wheels. Again remember, its easy to figure out what will clear coilovers or clear a wheel hitting your chassis. BUT it wont necessary mean the wheel will be flush or be sticking out too much.

THE FRONT (Lip part) OF THE WHEEL IS CALCULATED DIFFERENTLY FROM THE BACK
 
6'+54 or lower
6.5'+45 or lower
7'+40 or lower
7.5'+34 or lower
8'+30 or lower
8.5'+24 or lower
9'+20 or lower

front wheels only, rears should have more clearance for coilovers.
 
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just to let you know, a 17x10 wheel is not 254mm wide so centerline is not 127mm.

you need overall width for centerline and offset measuring, which in the case of a 10' wheel will probably be 11'.

and there's no reason to convert to mm until after you subtract centerline from backspace, which would give you the offset. it's pointless to convert backspace, overall width and centerline to mm since most people here are in the US and use inches to measure unless they are just trying to be jdm.
 
*case point and said.
a 10' wide wheel is not 10' physically.

roughly 11' yes.

i measured a 9' wheel, hair under 10'.

we all know americans seldom use mm.
so heres a inch table: