Car Physics Problem

Game Mechanics
#1

Hello. I have started developing a racing game, so I started by implementing the car physics by the guide of “Marco Monster”, as well as a little “debug screen”. It is partly german though, so here a little translation:
“Geschwindigkeit”=“Speed”
“LW-Kraft”=“Air resistance force”
“RW-Kraft”=“Rolling resistance force”
“Beschleunigung”=“Acceleration”

Anyways, I’m having a few problems here. First of all, I’m tracking the wheel speed separately, this works pretty well for high speeds, but at low speeds it just spazzes out. Also, if you’re not accelerating, the wheels decelerate wayyy to fast. The third problem is that the maximum speed is about 180 km/h (or 112mph) in 4th gear, because in 5th and 6th gear, the car continues to decelerate (This is pretty logical, considering the engine torque goes down, because of that the driving force goes down, so the air resistance is greater than the driving force, which decelerates the car, but that shouldn’t happen, correct?)

Here is the most important part of the source:

public void update(float delta, boolean accelerating) {

	float speed = v.length();

	Main.renderStringFormatted(29, "%d", (int) rpm);
	Main.renderStringFormatted(30, "%d", gear + 1);
	Main.renderStringFormatted(0, "Geschwindigkeit: %.1fkm/h", speed * 3.6);

	rpm = wheelSpeed * vehicle.getGearRatio(gear) * 30 / Constants.PI;
	if (rpm < vehicle.getMinimumRPM())
		rpm = vehicle.getMinimumRPM();
	if (rpm > vehicle.getMaximumRPM())
		if(shiftGearsUp())
			rpm = wheelSpeed * vehicle.getGearRatio(gear) * 30 / Constants.PI;
		else
			rpm = vehicle.getMaximumRPM();

	float Fdrive = vehicle.getEngineTorque(gear, rpm)
			/ vehicle.getTireDiameter();
	Vector2f Ftraction = accelerating ? dir.mul(Fdrive) : new Vector2f();
	Vector2f Fdrag = v.mul(-vehicle.getShapeDrag() * speed);
	Vector2f Frr = v.mul(-vehicle.getShapeDrag() * 30);

	Vector2f Flong = Ftraction.add(Fdrag).add(Frr);

	Main.renderStringFormatted(1, "LW-Kraft: %.1fN",
			Fdrag.lengthSigned(dir));
	Main.renderStringFormatted(2, "RW-Kraft: %.1fN", Frr.lengthSigned(dir));

	float Wr = vehicle.getBackTireLoad(Flong.lengthSigned(dir)
			/ vehicle.getMass())
			/ (vehicle.getMass() * 9.81f);
	Main.renderStringFormatted(5, "Load V: %.2f%s", Wr, "%");

	Flong = Ftraction.mul(Wr);

	Vector2f Ffriction = new Vector2f();

	if (speed > 0) {
		float slip = (wheelSpeed * vehicle.getTireDiameter() - speed)
				/ speed;
		Main.renderStringFormatted(4, "Slip: %.2f", slip);

		Ffriction = Ftraction.mul(Math.min(16.6666667f * slip, 1));
	}

	Flong = Flong.add(Frr);
	float Tfriction = Ffriction.lengthSigned(dir) * vehicle.getTireDiameter();
	float Tdrive = Flong.lengthSigned(dir) * vehicle.getTireDiameter();
	float Ttotal = Tdrive - Tfriction;
	float aa = Ttotal / vehicle.getRearTireInertia();
	wheelSpeed += aa * delta;
	Main.renderStringFormatted(6, "Tfriction: %.2fNm", Tfriction);
	Main.renderStringFormatted(7, "Tdrive: %.2fNm", Tdrive);
	Main.renderStringFormatted(8, "Ttotal: %.2fNm", Ttotal);
	Main.renderStringFormatted(9, "Angular acc.: %.2frad/s^2", aa);
	Main.renderStringFormatted(10, "Wheel vel.: %.2fkm/h", wheelSpeed * vehicle.getTireDiameter() * 3.6);
	
	Flong = Flong.add(Fdrag);
	Vector2f a = Flong.div(vehicle.getMass());

	Main.renderStringFormatted(3, "Beschl.: %.2fm/s^2", a.lengthSigned(dir));

	v = v.add(a.mul(delta));
	if (dir.dot(v) < 0)
		v = new Vector2f();

	pos = pos.add(v.mul(delta));
}

And here is a SSCE: download (Source code/debug scripts/LWJGL/runnable jar included, start the debug.bat for your system)

Thanks for your time.