See pic. These are stock rims off of a 78'. The tape reads 15" from the edge of the rim to the other edge or about 14" inside the edge. What's the correct way to measure the size of the rim? Or, saying it another way, what is the stock rim size on the 78'?

My understanding is that the tire size is the inner diameter of the tire and that there's a bit of overlap with the rim at the bead. My 14" wheels also measure a total diameter of about 15". I'm assuming that you have 14" tires on those wheels so that tells you all you need to know.

http://www.rsracing.com/tech-wheel.html

You have a tire mounted . Just look at the tire . It will tell you . there are three numbers the last is the rim size. 195 70 ( 14) 14'' wheel