Focus On Rotation's Per Minute, Not Simulated "Miles Per Hour"
As a certified indoor cycling instructor and road cyclist I can tell you that simulated speed on an indoor bike is generally a bad measure as you generally aren't going to know how it is calculated and it doesn't have a good relationship with technique. Certification training instructs us that the RPM range given to our students generally shouldn't go outside of 60-100, with an admission that serious cyclists will often want to raise this to an 80-120 range (in other words I instruct the serious cyclists to attempt working at the given range +20).
The key point being that at lower RPM's you are adding potential for injury and at higher RPM's you are probably over-spinning (thus not really working AND adding potential for injury). In road cycling their is an added benefit to a good cadence that keeps you from losing energy on your strokes.
My instruction to you would depend on what you want to accomplish:
I would tell to mix it up and aim for high RPM's on simulated sprints, and hit the lower RPM's on simulated hill climbs. The experience of getting both in would prepare their body not only for the mixed experience of the road, but to actually get adjusted to using efficient RPM's.
HIIT for primarily cardiovascular fitness
For HIIT focused primarily on cardiovascular fitness I would say to focus on the higher end of the range, probably 90-100.
HIIT for cardiovascular power
If you were hoping for a bit more power and don't really plan on ever brining it to the road then 70-80 RPM's is a solid goal that still keeps the process in the low-injury low-impact workout realm.