Usage: SetVehicleSpeed(vehicleid, Float:speed);

stock SetVehicleSpeed(vehicleid, Float:speed)
{
    new Float:x1, Float:y1, Float:z1, Float:x2, Float:y2, Float:z2, Float:a;
    GetVehicleVelocity(vehicleid, x1, y1, z1);
    GetVehiclePos(vehicleid, x2, y2, z2);
    GetVehicleZAngle(vehicleid, a); a = 360 - a;
    x1 = (floatsin(a, degrees) * (speed/100) + floatcos(a, degrees) * 0 + x2) - x2;
    y1 = (floatcos(a, degrees) * (speed/100) + floatsin(a, degrees) * 0 + y2) - y2;
    SetVehicleVelocity(vehicleid, x1, y1, z1);
}