After calling around a few more places, I think I'm surrendering to the same conclusion: Just live with it. Every solution is pricey or involved or both. I thought a little adapter module could be had for say ~$100 with easy install. No such luck.
The math approach works, but my brain likes a look-up table corresponding to notable speed limits:
- Shows "75" so it's really -> 65mph
- Shows "40" so it's really -> 35mph
- Shows "30" so it's really -> 25mph
- Etc.
Whatever works. And for total miles on the odo, oh well, will just have to explain and demonstrate it to any future buyer.