Car acceleration

Reading this post and thread may help:
http://www.gamedev.net/community/forums/viewreply.asp?ID=3560185
http://www.gamedev.net/community/forums/topic.asp?topic_id=542915

The speed is not limited by forces, it is limited by the engine's maximum rpm (assuming the engine torque is capable of overcoming resistance forces, and it almost always can). At some point the engine cannot go any faster, if you want the car to go faster, you'd have to disconnect the engine, which means no more torque applied to the wheels.

The car is accelerated by the torque of the engine being converted to linear motion through the wheels.

Your forward key will apply the torque. Say you want a real basic engine simulation where it has a max torque which is scaled by the throttle. The throttle is your forward key. You can also incorperate a transmission with one or more gear ratios between the engine torque and wheel torque.

float engineSpeed = wheelSpeed / gearRatio;
if (engineSpeed < 0) stalled();
if (engineSpeed > maxEngineSpeed) exploded();

float torqueEngine = maxTorque * throttle;
float wheelTorque = torqueEngine * gearRatio;
carForce = wheelDirection * wheelTorque / wheelRadius;