Very first attempt at gear locking in GTA V

Gears.cs

using System;
using GTA;
using GTA.Native;

public class Gears : Script {
    Random random = new Random();

    public Gears() {
        Tick += OnTick;
        Interval = 0;
    }

    void OnTick(object sender, EventArgs e) {
        Ped player = Game.Player.Character;
        Vehicle vehicle = player.CurrentVehicle;

        if (player.IsInVehicle() && vehicle.Model.IsCar) {
            ulong address = vehicle.GetAddress;
            uint gears = vehicle.CurrentGear;
            float rpm = vehicle.CurrentRPM;
            float clutch = vehicle.Clutch;
            float throttle = vehicle.Throttle;
            float turbo = vehicle.Turbo;
            uint currGear = (0xFFFF0000 & gears) >> 16;
            uint nextGear = 0x0000FFFF & gears;

            
            // Park/Reverse. Gear 0. Prevent going forward in gear 0.
            if (currGear == 0 && !Game.IsControlPressed(0, Control.VehicleBrake) && vehicle.Speed < 1.0f) {
                vehicle.HandbrakeOn = true;
                vehicle.BrakeLightsOn = true;
            }
            // Forward gears. Prevent reversing.
            else if (currGear > 0 && !Game.IsControlPressed(0, Control.VehicleAccelerate) && Game.IsControlPressed(0, Control.VehicleBrake) && vehicle.Speed < 1.0f)
            {
                vehicle.HandbrakeOn = true;
                vehicle.BrakeLightsOn = true;
            }
            else
            { 
                vehicle.HandbrakeOn = false;
            }
            

            // Game wants to shift up. Triggered at high RPM, high speed.
            // Desired result: high RPM, same gear (currGear)
            if (currGear < nextGear) {
                vehicle.CurrentGear = currGear | (currGear << 16);
                vehicle.CurrentRPM = 1.6f;
                vehicle.Clutch = 1.0f;
                vehicle.Throttle = 1.0f;
            }

            // Game wants to shift down. Usually triggered when user accelerates
            // while in a too high gear.
            // Desired result: low RPM or stall, even. Stay in current gear!
            // Otherwise: force car to drive anyway to keep RPM up.
            // Current: Game cuts engine power
            if (currGear > nextGear) {
                vehicle.CurrentGear = currGear | (currGear << 16);
                vehicle.Clutch = 1.0f;
                vehicle.Throttle = 1.0f;
                /*if (currGear == 2) {
                    vehicle.CurrentRPM = Game.GetControlNormal(0, Control.VehicleAccelerate) * 0.40f;
                }
                else
                {
                    vehicle.Clutch = 1.0f;
                    vehicle.Throttle = 1.0f;
                }*/
            }
            
            // Shift up
            if (Game.IsControlJustPressed(0, Control.VehicleDuck)) {
                vehicle.CurrentGear = currGear + 1 | (currGear + 1 << 16);
                vehicle.Clutch = 0.1f;
                vehicle.Throttle = 0.5f;
            }

            // Shift down
            if (Game.IsControlJustPressed(0, Control.VehicleSelectNextWeapon)) {
                if (currGear > 0) {
                    vehicle.CurrentGear = currGear - 1 | (currGear - 1 << 16);
                    if (vehicle.EngineRunning == false) {
                        vehicle.EngineRunning = true;
                    }
                }
            }

            // Trying stuff
			if (Game.IsControlPressed(0, Control.VehicleAim)) {
                vehicle.Throttle = 1.0f;
                vehicle.Clutch = 1.0f;
                vehicle.CurrentRPM = 2.2f;
            }
			
            string info = "RPM: " + rpm +
                        "\nCurrGear: " + currGear +
                        "\nNextGear: " + nextGear +
                        "\nClutch: " + clutch +
                        "\nThrottle: " + throttle +
                        "\nTurbo: " + turbo +
                        "\nAddress: " + address.ToString("X");

            Function.Call(Hash.SET_TEXT_FONT, 0);
            Function.Call(Hash.SET_TEXT_SCALE, 0.4f, 0.4f);
            Function.Call(Hash.SET_TEXT_COLOUR, 255, 255, 255, 255);
            Function.Call(Hash.SET_TEXT_WRAP, 0.0, 1.0);
            Function.Call(Hash.SET_TEXT_CENTRE, 0);
            Function.Call(Hash.SET_TEXT_DROPSHADOW, 0, 0, 0, 0, 0);
            Function.Call(Hash.SET_TEXT_EDGE, 1, 0, 0, 0, 205);
            Function.Call(Hash._SET_TEXT_ENTRY, "STRING");
            Function.Call(Hash._ADD_TEXT_COMPONENT_STRING, info);
            Function.Call(Hash._DRAW_TEXT, 0.01, 0.5);
        }
    }
}

vehicle.cpp

System::UInt32 Vehicle::CurrentGear::get()
{
	unsigned char *const address = reinterpret_cast<unsigned char *>(Native::MemoryAccess::GetAddressOfEntity(Handle));

	int offset = 0x7a0;

	return address == 0 ? 0 : *reinterpret_cast<const System::UInt32 *>(address + offset);
}	
void Vehicle::CurrentGear::set(System::UInt32 value)
{
	unsigned char *const address = reinterpret_cast<unsigned char *>(Native::MemoryAccess::GetAddressOfEntity(Handle));

	int offset = 0x7a0;

	*(address + offset) = value;
}
float Vehicle::Clutch::get()
{
	unsigned char *const address = reinterpret_cast<unsigned char *>(Native::MemoryAccess::GetAddressOfEntity(Handle));

	int offset = 0x7E0;

	return address == 0 ? 0 : *reinterpret_cast<const float *>(address + offset);
}
void Vehicle::Clutch::set(float value)
{
	System::UInt64 address = Native::MemoryAccess::GetAddressOfEntity(Handle);

	int offset = 0x7E0;

	*reinterpret_cast<float *>(address + offset) = value;
}
float Vehicle::Turbo::get()
{
	unsigned char *const address = reinterpret_cast<unsigned char *>(Native::MemoryAccess::GetAddressOfEntity(Handle));

	int offset = 0x7F8;

	return address == 0 ? 0 : *reinterpret_cast<const float *>(address + offset);
}
void Vehicle::Turbo::set(float value)
{
	System::UInt64 address = Native::MemoryAccess::GetAddressOfEntity(Handle);

	int offset = 0x7F8;

	*reinterpret_cast<float *>(address + offset) = value;
}
float Vehicle::Throttle::get()
{
	System::UInt64 address = Native::MemoryAccess::GetAddressOfEntity(Handle);

	int offset = 0x7E4;

	return *reinterpret_cast<float *>(address + offset);
}
void Vehicle::Throttle::set(float value)
{
	System::UInt64 address = Native::MemoryAccess::GetAddressOfEntity(Handle);

	int offset = 0x7E4;

	*reinterpret_cast<float *>(address + offset) = value;
}
System::UInt64 Vehicle::GetAddress::get()
{
	const System::UInt64 address = Native::MemoryAccess::GetAddressOfEntity(Handle);
	return address;
}

This is a ScriptHookVDotNet file.
vehicle.cpp was modified from ScriptHookVDotNet to access more CVehicle members. If, for some reason, you want to re-build the version I started everything with, SHVDN version 2.something was used. Version 3 onwards is fully C# for game code.