No, no I could not just look and guess. I did have to measure. And I did.
Here is the process: as the imaginary circle diameter cannot be measured directly with a five bolt pattern (it can with a 4 or 6 bolt pattern) I used a digital vernier to mesure the distance between two alternate bolt centers: not adjacent, skip one and go to the second bolt. THis is the largest distance that can be actually measured on this setup.
Now here is the calculation, for those who like to peek at a little bit of trigonometry.
a) a triangle is formed with summits a,b,c opposing sides A,B,C : summits a and b are the center of alternate bolts, and the third is the center of the hub. The angle formed at the center with the radius (between sideA and side B) going to each alternate nut is 2/5 of a circle, hence 144 degrees.
b) A and B, the two radius, are the alleged half diameter of the bolt circle, namely either 114.3/2 or 115/2
c) the length of the third side, the distance between the center of alternate bolts, is calculated with the equation C = sqr(A*A + B*B -2*A*B*cos(c))
..you can do it with the scientific calculator or use triangle calculators on the net.
Based on alleged bolt circles, we should have
C = 108.706 mm for 114.3mm (4.5in) bolt circle or 57.15mm radius
C = 109.3715 mm for 115mm bolt circle or 57.5mm radius
geometry
www.avigex.ca/xport/hubcircle.jpg
I trained the digital vernier with measurements using alternatively inside and outside measurement prongs to alleviate visual illusions when measureing from the stud centers. I also measured outside of the studs, and inside of the studs and averaging the 2 readings for the center..
inside measurement
www.avigex.ca/xport/ver5s.jpg
outside measurement
www.avigex.ca/xport/ver2s.jpg
I took tens and tens of measurements to extract an average reading from the sum of all readings thus taking care of over and under measurements. Measurements were consistent within a very small margin of variation . Measurements were conducted on both front and rear hubs on the car, and on three other hubs on the bench, two front and one rear ( taken off the car for bearing hum, a classic on the TS) .
So here is the punch. Measurements fell consistently within 107.9 and 108.9, most near to 108.6 with a few exceptions over 109, thus giving an average of 108.65, very near to 108.7 that would correspond to 4.5 in bolt circle. I extended the vernier to 109.3 and in that condition, the prongs of the vernier fell ostensibly outside the center of the studs. So conversely, the 115mm convenience label would turn out to be even less than 114.3, as tested on seven GM wheel hubs that GM labels as 115 One of the hubs on the table, though, visibly from a different manufacturer, was just a little over all the others and provided an average (of ten readings) at 109.077mm, hence between 108,7 and 109.37, and corresponds to almost midway between 114.3 and 115.
Bottom line: none of the measurements, even if we isolate the highest readings, prove the 115 label. "GM proprietary" 115 label IS actually closer to standard 4.5in 114.3mm.
WIth that number of takes, the residual error in measurement is dramatically small....another book on statistics would have to be opened here....The only way these measurements could have been significantly false would have to involve a faulty digital vernier but do not have any doubts.....I will go to that length and test another vernier agaisnt mine. Any discrepancy WILL be reported.......