The speed of the vehicle is limited by the maximum speed that the wheels can spin round and that is limited by the gear ratio and the maximum engine rotation speed. The theoretical limit is as follows:
maxWheelRotationSpeed(currentGearRatio) = maxEngineOmega/(currentGearRatio*finalRatio);
maxVehicleSpeed(currentGearRatio) = maxWheelRotationSpeed(currentGearRatio)*wheelRadius;
These are theoretical limits because engine power is lost to engine damping, wheel damping, rigid body damping and imperfect translation of engine torque to tire force. Some power is lost to the simulation itself due to the step size of the integration.
If you want to increase the maximum speed then you would need to either increase the maximum rotation speed of the engine or decrease the gear ratio. I would recommend adding an extra gear that has a low gear ratio and then toggling the number of gears on the vehicle so that the extra gear is either accessible or inaccessible depending on your game logic. You could obviously do the same by toggling the max engine rpm between two values depending on your game logic. However, that might sound strange if you have audio that reflects the engine rotation speed.