Well ,that was so many balls..... Good job, you did well. I don't feel any lag. I say 8 of 10. You may next time make bigger piston and cylinder for better performance.
How to fix gearbox - event you make infinity friction, it still not be infinity you have to make heavy circles to make better friction. Make Circles 100 kg/m(2). If then your'e car go slow then you have to make more powerful engine. Good luck