As all the fields which control your speed-gauge are pre-calculated, I can't see the logic on why it breaks down when you change to a different month. I suspect the "manual" colouring set up
is set up for the first month, and some of these need different assignments for the different month.
I would always strongly advise against using Speed Gauges, as there are much better ways (Bullet Charts) for showing Actual vs Target (and similar).
Below is a blog (albeit tongue-in-cheek) on how to create these using background images (which won't require such bespoke colour assignments for each situation)...this blog also has some links to articles explaining why speed gauges are a pretty bad way to visualise this
and here is another article critiquing Speed-gauges vs bullet-charts.