The Speedometer is a feature in earlier Mario Kart games that shows how fast a kart is driving. The Speedometer can only reach it's limit while on 150 cc engine class. It can even go OVER the max when using a Mushroom or Boost Pad.
First introduced in Mario Kart 64, it was a second option that could be toggled on in place of the mini-map (third option was distance view). It appears on the bottom right hand corner of the game screen in Mario Kart: Double Dash!!. In both games it is only available in single-player mode (or co-op Grand Prix). It also appears in the Time Trial mode for Mario Kart: Super Circuit.