WAHOO!!!
Mission successful!
I put the ratio adapter in, fired up the GPS speed app on my phone, and took a spin. At 30mph actual, my speedo reads right about 31mph. At 60mph actual, my speedo reads 62-62.5mph.
I couldn't be happier, and I don't think I can expect much closer accuracy than this.
What are the odds that a box of miscellaneous parts from my father in law would contain the exact adapter I need??!! Especially with my tire/gear ratio combo..
Thanks again for you help with the math Rich. Without your input on what ratio I needed, I wouldn't have known what to look for.