measuring OFFSET -
LOOK AT THE FRIGGIN' WHEEL before you bother measuring
http://carpron.com/albums/album16/DSC01493.sized.jpg
many wheels have all the pertinent info stamped on the back somewhere, generally between the mounting points or on the back of the spokes. oddly, some of the pricier makers like volk use stickers that often get taken off for whatever reason. if your sticker is gone or there's no info on the back, start measuring (take the wheel off first).
you need:
1. something to lay across the back of the wheel. flat, not bent, doesn't bend. something with definite edges is good. I used a large ruler sideways so I wouldn't have to worry about it flexing.
2. some change if the tires are still on the tires (to put between the flat thing and the wheel edge since many tires overhang the wheel and will screw up the measuring). I used a quarter and a penny which equal 3mm in width (measure your own change just in case - especially folks not from the U.S.
)
3. a measuring implement ....... I used a tape measurer, but you could use just about anything and just mark it and measure the marks with a ruler later if that seems easier to you.
4. paper and pen if you're like me and forget numbers all the time.
Alright, time to measure.
1. Measure the width of the wheel from outermost edge to outermost edge. The wheels I measured came out to 10" on the tape measurer. The actual width of the wheels is 1" thinner than measured due to the flanges being 13mm wide on either side. So, the number I write down is 9".
2. lay the flat thing across the outermost portion of the wheel. if there's tire in the way, use the change (quarter and penny in my case) to find the error (3mm in my case).
3. measure from the mounting face of the wheel (the part that actually touches the hub on the car) to the flat thing. the number I got here was 151mm.
now for simple math.
1. take the width and divide by 2. 9 divided by 2 = 4.5" for me.
2. convert half the width to mm. 4.5" = 114.3mm
3. subtract that from the total measurement (151mm in my case). 151 - 114.3 = 36.7
4. subtract the error (if any) from 36.7 - 3 (the quarter and penny) = 33.7mm
5. subtract 13mm for the backside flange that was included when you measured. 33.7 - 13 = 20.7mm
offset = 20.7 according to my measurements. actual offset (stamped on the back of the wheel) is 20 so good enough considering my crude implements. Use quality tools and measure multiple times to ensure accuracy. I was only .7mm off with a camera in one hand so if you have somebody there to help you and are careful it should be no problem.
13mm is the width of J or JJ flanges. There are some exceptions like JK maybe ..... haven't seen a width measurement for that flange and HRE uses a different size (unknown to me) also. CCW and some other custom wheel makers may also use something slightly different. Almost any wheel you run across will be J or JJ though.