+40 offset > 5.5 Backspacing

I figured its worth a note, any time B.S. is mentioned for the X.. 5.25" is found 99% of the time. I finally took a ruler to the 7" rims, and came up with 5.5" .. I did some number crunching, and it does indeed work out to 5.5".
Example
Known offset of stock rims is +40, listed in the manual,
AR 767 7" rims is listed as 0 Offset/3.95 B.S.
40(difference in offset) divided by 25.4 conversion = 1.57"
3.95 + 1.57 = 5.52 Backspace of stock rims.
Not anything important.. but something when comparing to 8" rims.

To what point were you measuring? I'm told that the backspacing is measured to the inside of the lip, not the outside. That's about 1/4" difference.

Brent

The diagram I have shows it to the outside of the lip ( I couldn't find the diagram Ian always posts to confirm ) .. but once I calculated the numbers from other rims with both BS & OS listed.. it works.
It also explains why you couldn't match offset to backspace by simple calculation i.e. 7" rim 0 offset should equal 3.5 backspacing.. I have yet to see a rim with this number ( the thickness if the lip is added to it ) therefore 3.95 like the AR 767's

It seems OEM measurements are to the bead, and not the actual edge of the wheel. OEM usually doesn't bother with the backspacing measurement either, but rather just offset since it's much more accurate.

The 7" wheel is actually 8 inches edge to edge. It's 7 inches from bead to bead.

Hence why I always reference offset instead of backspacing...

Brent

I just posted because of the move to 8" rims by some.. and offset numbers are not comparable.. found an ABC to it all.. http://www.usacomp.com/Offset.htm

So then, what is the backspacing for the stock 17x8 wheels (as I have)??? 5.25? 5.5? more?

Yes, I know I could go and remove a wheel and take the measurements myself but I haven't had the time.

Thanks for any info.

Also, there's an AR wheel I'm interested in. It has 19mm offset with a 5.5" backspacing.

By attempting to emulate your calculations...

40-19 = 21 divided by 25.4 = .83"
5.5 + .83 = 6.33"

Therefore, if factory backspacing is, for example, 5.5" then these wheels (at 6.33") will not fit. Am I doing this correct?

Actually you do not need to do the measurements your self as it is stamped on the inside of the wheel.

The back spacing on the 17x8 was about 6.25" if I can remember correctly.

Alan

So by using my calculations, the wheels would not fit...

6.25 factory
6.33 aftermarket

Correct?

Incorrect. I think your math is wrong. If the rims only have a 19mm offset they will fit but you may need to trim the front mud flap a little to maintane the stock turning radius.

Call your dealers part department and double check the specs on the 17x8 rims.

(with a 19mm offset you would take 9/2 + 19mm gives you about a 5.25 backspaceing meaning the tire will stick out about 1" over the stock rims.)

ALAN

Thanks Alan.