I just measured mine and from the wheel mounting surface to the inside edge of the bead flange is 3 1/2". That would equate to a negative 1/2" offset.

1973 L-82 4 spd

Greetings to those who may be interested.  The December 2021 issue of Corvette Magazine has a fantastic writeup on Roll Models, (wheels), for C1-C3 steel and aluminum wheels. It may answer All of your offset and back spaced questions. 

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I don't get that mag anymore.

The article says that the 15x8-in rally wheel had a -11 mm offset in a 3 1/2" backspacing. Basically what I measured last night.  Not sure why they mixed measurement systems but it is what it is.

1973 L-82 4 spd

So that is 3.5 inches from the bead?  That would make it 4.0 inches from the lip....I guess.

It's from the inside vertical surface of the wheel that the bead rests against.  I used to measure from the flange to the mounting surface and that gave me 0 offset.  Need to measure the wheel and see where the 8"  measurement is.  Tomorrow's project.

1973 L-82 4 spd

The 8-in measurement is from the inside of the flange across the wheel to the inside of the flange. So my previous measurements re: offset and backspacing are correct.  

1973 L-82 4 spd

Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

So now I'm reading that I should have my wheels balanced lug centric and not hub centric.  That may be why I can't get rid of this little shake at 52-54 mph.

Also, in looking for wheels at all the aftermarket places, their backspacing measurements are all over the place.

I measured 3 of my wheels and from the bead it is 3 5/8" and from the lip it is 4".  The 3 5/8" goes along with Adams Apple's "B" measurement as the steel is probably about 1/8" thick.

Geez Louise.

Have you considered a road force balance?

1973 L-82 4 spd

Yeah, I can do that, but with steel wheels, lug centric balancing is better than hub centric as the hole in the middle isn't as precise as a machined alloy wheel which is balanced with a cone centering the center hole.

What is annoying is Ecklers sells the Corvette 15x8 rally wheel with a "correct" 4.25 backspacing but then says a 1/4" spacer is needed.

Mid America sell the same factory "correct" rally with a 4.25 backspacing and never mentions a spacer.

And they are obviously measuring backspacing from the wheel lip and not the bead.

This is maddening.