Just to clear a few things up! 😀
* When I said that a bus speed of 60 or 66 could not be achieved, I was referring to the FreeTech SiS chipset-based 486 mobo. However, it actually makes no difference, because I'm fairly sure that this also applies to the 486-GIO-VT mobo as well. In fact, the only 486s that have an undocumented bus speed of greater than 50 appear to be late era PCI UMC chipset-based mobos.
* The speed jumper settings of 60 and greater for the 486-GIO-VT on that Stason webpage are for internal speeds. For example, a DX4-100 has an internal speed of 100, requiring an external speed of 33. External speeds are the bus speeds, and they can only go up to 50 on most 486s. You'll sometimes see the internal speeds listed on Stason as i50, for example. Sure, it does list 4MHz in that table, and that would be for the bus.
* "Well, my AMD DX-40 did manage to boot at 60Mhz and 66 Mhz, so the newer DX-50 might be perfectly capable of that." I bet the board you tried that DX-40 on was a PCI 486 UMC chipset-based mobo. Yes, the DX-50 might be perfectly capable of that, but not in a SiS chipset based 486 mobo, because it doesn't support the necessary bus speed.
* Returning to the topic of the 486-GIO-VT mobo, and the idea that it might support 100 bus speed. What 486 CPU listed at the top of that Stason webpage would POST at such a speed? I can't think of one. In fact, I can't think of any 486 CPU that would work at that speed.