Ron, thanks for posting this thread. Darkside is very intriguing to many and the more people share their experience with rest of us, the better.
I'm also interesting to try it out and while doing extensive research, I found this article from the righter for Iron Butt Association - Examining the “Dark Side”
Can you please look at it and share your thoughts, the article examines both, pros and cons of a darkside.
What really caught my eye there, that 15" MC rim is 0.1" larger than car rim. 17" MC rim is 0.1" smaller than car rim. While only 16" MC rim matches size of car rim... So, maybe that was the reason your car tire installed easy, as you said, maybe too easy? And that's why 16" rim of GL1800 successfully darksiding, due to better fit?