Unflyable? Because you are getting 2,300RPM on the ground? In what way is it unflyable?
Note that a maximum of 2,300RPM on a ground runup is realistic for a C172, both in real life and in the sim. With the aircraft stationary, you will never get the aircraft to attain significantly higher RPM or get closer to maximum RPM. The propeller is designed to function within the normal operating air speed range. When stationary, you can compare it to you trying to get your car engine to accelerate in top gear at low speed: the engine ends up labouring. Once you start your takeoff run, as the speed increases, so the RPMs increase. When climbing at normal climb speed of around 74kts, RPMs will be around 2,500 at sea level. If you level out close to sea level and keep full throttle, as the aircraft accelerates, the RPMs will increase up to the maximum of 2,700RPM.