u1923

i found a set of used ccw i want to get but the seller doesn't know the offsets. is it possible to find out the equivalent offsets from backspacing measurements:

6.25" backspacing on 18x9.5
and
6.75" backspacing on 18x11

someone please help me out. thanks

5sp4banger

backspacing is the measurement from the plane of the rear lip to the plane of the wheel hub. within this measurement is the offset. offset is the measurement from the plane of the CENTERLINE to the plane of the wheel hub. so subtract half of the rim width from the backspacing value and then multiply by 25.4mm.

18x9.5: 6.25 - 4.75 = 1.5 ~> 38.1mm offset
18x11: 6.75 - 5.5 = 1.25 ~> 31.75mm offset

u1923

THANK YOU 5sp4banger!! you'd just made my day! one more question

what do you (or anyone) think about this set up on a stock sc400? will i need some serious fenders rolling on the rear? thanks

SoCalSC4

5sp is on the right track, but the offsets are wrong!

People forget that a 9.5" wheel is really 10.5" wide. The wheel width dimension does not include the inner & outer bead seat thickness, which is 1/2" per bead. Hopefully, you considered this when measuring the car for wheels!!!!!!!!!

18x9.5 = 10.5" overall. The centerline of a 10.5" wide wheel is 5.25"
Your desired backspacing is 6.25", leaving a difference of 1.0"
Multiply 1.0" x 25.4 to convert to millimeters
18x9.5 +25.4mm

18x11.0 = 12.0" overall. The centerline of a 12.0" wide wheel is 6.0"
Your desired backspacing is 6.75", leaving a difference of .75"
Multiply .75" x 25.4 to comvert to millimeters....
18x11.0 +19.05mm

I hope it's not too late to fix your wheel order... otherwise, you'll be in the market for some spacers...

u1923

thanks! socalsc4, that was very helpful. wow, those offsets are way too aggressive for my bone stock sc

5sp4banger

if that's the way it works then i didn't know but...that's confusing as the widths should have been stated as 1" wider. rather than guesstimate, check for the offset if you can. good luck with the search!

SoCalSC4

It's confusing, but that's the wheel & tire industry for you. The tire manufacturers are concerned with how the tire 'beads-up' on the wheel, so the wheel width measurement is given as 'inner bead to inner bead,' or between the flanges. It's pretty standard. A 16x7 is really 8" wide overall. A 19x13 is really 14" wide overall.

Also, the actual wheel diameter is larger than the stated size... again, the tire manufacturers are concerned with the diameter of the bead-seat area- a 19" tire has to fit a 19" wheel, and so-on. But you need a flange to locate ot 'hold' the tire on the wheel. A 19" wheel (for example) really measures about 20.5" outside-to-outside.

You measure a wheel diameter & width in inches, backspacing in inches, offset in millimeters, tire diameter in inches, tire width in millimeters, and sidewall height is a ratio. Clear as mud, right?

If it wasn't confusing, I wouldn't have a job!!!!!!