Speedometer and gauges

I don't know of any reason the amp gauge would affect the fuel gauge.
A higher than normal gas gauge reading indicates low resistance from the gauge to ground. A bad connection would give a higher resistance and cause the gauge to indicate low fuel - not higher than before. That is strange. Are you sure the tank is half full now? I'd go to the gas station and top off the tank just to make sure.

Do you know if the temp gauge is working normally? The reason I ask is that they are both powered by the output of the instrument voltage limiter.

Don't know about the speedometer gear but hopefully someone else here will be able to help you with that.