It's fixed as it's a Tachymeter scale rated for cycling i.e if it takes a minute to cycle 1 mile/kilometre you're doing 60MPH/KPH on the outer scale. The middle line is to measure the speed if it takes 2 minutes its 30MPH/KPH and the inner line is for me counting how many hours it would take 😂 Anything in between becomes self explanatory.