Ugh, gear inches, I prefer to know what speed I can travel at 90 RPM. I find that much more useful and easier to remember and explain. Anyone who has driven a car with a tach can easily understand this concept.
For me I use a low gear that is somewhere between 13-15km/hr@90rpm, and a high gear of about 30-35km/hr@90rpm. If I were to encounter extreme terrain, a lower gear that can sustain 6-8km/hr@90rpm may be necessary.