Hello and welcome!
The RX3 with fuel injection is very sensitive to valve clearance. Be sure you have that adjusted correctly.
RX3 also have sprockets that are better for high speed. For trails most riders have changed to 13t front sprocket, and some have also changed the back sprocket to 46t from stock 44. Really makes the power feel better.
I can not help you about the speedometer, I don't look at mine.
You can make the fuel gauge read better by bending the arm for the float. You will find instructions somewhere here.
Good luck, enjoy your RX3!