I’m buying wide-five wheels for my Vintage Speedster. One company (CIP1) offers them with “3 3/4” back spacing”, the other company (vintage speedster parts) offers them with “4.25 offset.” What do these mean and which is right for my car?
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Backspacing is the distance of the mounting surface from the inside rim edge (where the tire bead sits). 3 3/4" on a 5 1/2" rim means the mounting surface is 1" to the outside of the center of the rim and 1 3/4" from the outside rim edge. While the chart below doesn't exactly have the wheel you're looking at, it does show the backspacing/offset relationship as it applies to different Fuchs 911 5 spoke wheels.
You can see how, when the 5 1/2x15 was made they added 1/2" to each side so the mounting surface stayed the same distance from the middle of the wheel so the offset stayed the same while the backspacing (or backsize, as it's called here) increased by 1/2". When the 6x15 was made the 1/2" increase was solely on the outside, so now the backspacing stayed the same while the offset (and thus the middle of the rim) increased. With the 901 numbered 15x7 (uber expen$ive 911R wheel) the full inch was added to the inside (so it would tuck under the straight sided 911 rear fenders) and now both the backspacing and offset measurements both increase. When the regular production 7x15 (911 part #) was introduced, the inch was added to the outside of the 6" wheel, as the rear panels on the car were modified to accommodate the increase in track (possibly to make the car handle better?)
Getting back to your situation- the 3 3/4" (95mm) backspacing of the CIP wheel gives 21mm offset (approx), and means the mounting surface is 1 3/4" (44.5mm) from the outside edge of the wheel instead of a little over the inch shown in the diagram of the 5.5" Fuchs. The 5 1/2" wheel from Vintage Speedster Parts can't have 4.25" offset (it isn't possible to have more than 2 3/4" (or 70mm) offset on a 5 1/2" wheel, as it's the measurement from the middle of the wheel to the mounting surface), so 4.25" (108mm approx) must be the backspace #. That means there's 1 1/4" from the mounting surface to the outside of the wheel. I would confirm this with VSP.
I would think that with short axles and tubes (the way VS Speedsters are built, I believe) either wheel would fit as there's not a lot of difference, but don't take my word for it. What's on the rear of the car now for wheel and tire sizes? How much room to the fender edges? What size tire are you thinking of putting on the back? I'm guessing 185's? If you're truly worried about fender clearance then the VSP wheels will tuck inside the furthest (by 1/2"), while if the CIP wheels will clear then you'll get a little more rear track out of the deal, and I'll bet there's enough room to widen those wheels an inch to the inside and run even wider 205's!
(think big, evil laugh here!)
And yeah, I know, I can never leave well enough alone (come on, you guys should know that by now!) Al
GREAT chart, Al...... Thanks!
This seems to be a concise easy to follow definition -
Touche, Greg! Way better than my long winded dissertation...
That's OK, AL...... I remember sitting in a meeting with Dick Egan, the "E" of EMC Corporation and I had just presented a lengthy explanation of where we were on the introduction of a new product line. I finished and was pretty proud of how complete it was and how everyone seemed to understand what it was all about.
Dick turns to me and says, "You can say less, with more words, than anyone else in this room..."
Talk about deflating a balloon......................... 
So what would be the suggested wheel for a CMC with the tight driver side running 185" and 4 bolt system/
And to my original question, what are you guys carrying on Vintage Speedsters with wide five 15 x 5.5s?
