You probably need a new one of these

It goes on the front axle nested against the front wheel. It’s notorious for the 4 tabs to break off allowing it it spin freely. It usually has an intermittent period where it works inconsistently before it fully breaks.

The bikes typically claim they are going 5-10 miles faster than they are. There are aftermarket speedometer healers that can fix it if you care enough or you can do a gixxer front end swap as they have a slightly larger diameter wheel which accomplishes the same thing.