The gears are for the odometer only.
The speedometer can't be adjusted via gearing.
When changing axle/differential ratio, you need an electronic converter box like YellowBox (on backorder) or Abbot & Costello ERA, which Gerry linked above. Th is will adjust both speedometer & odometer automatically, and works if you are not changing the speedo faceplate scale (i.e, if you change from a 160mph to 190mph faceplate, custom modifications are required - not DIY'able AFAIK).