Bottom line is, aside from safety concerns, you almost certainly cannot do it. I wrote this reply to a similar question on another forum:
I drove it in 2009 in my own car, heading the other way, from Peshawar - Kabul - Mazar - Termiz (and then right across Afghanistan). You can read about it here:
Stage 16 – Pakistan & Afghanistan: The Khyber Pass and Beyond [2/3] | EurasiaOverland
I had to get special permission to drive through Jamrud and Torkham tehsils.
At that time, going the way you want to go was really easy, the Afghans didn't care where you went, and as soon as you crossed into Pakistan, they gave you an escort and you didn't have to worry about any permits.
From what I understand now, the Afghans don't allow foreigners to drive to Torkham and cross the Khyber Pass. If you drive anything other than a Toyota you will probably stand out and get stopped, but with the right car and appearance you could probably get through.
Whether either side would let you through at the border itself, I am not sure, but you would be kind of screwed if they didn't. It did feel like the kind of border where you could negotiate - the Afghans at first did not want to let my truck in as I din't have a route pass, but some talking and everything was ok. If you're driving a junk car that you don't mind dumping, then you could always fly from Kabul.
Now, this makes a few assumptions:
You have a carnet de passages for your car in order to take it into Pakistan (it is not required for Afghanistan, but you will need for India) It will however make ditching the car difficult and expensive.
You have a Pakistani visa which allows you to enter by land.
You have a route pass for Afghanistan, or manage to talk you way in without one.
The BIG one is security. The situation in 2014 will be very different from what it was in 2009, and you will need to have very good info from people on the ground at the time. 2014 is not lined up to be a good year for Afghan tourism. The road from Kabul to Jalalabad and on to Torkham is one of the more dangerous in Afghanistan, and you need to know someone who is driving the road regularly.
Time wise, you would likely be looking at three days at the very minimum, one day to cross the Termiz border and get to Mazar, one day toKabul, then one more to Torkham. As far as I know, the Afghan police / army don't have the resources or motivation to escort foreign wanderers, and you'd probably be better off slipping through places unnoticed.
I assume this was just a 'what if' enquiry, and that you won't consider attempting it, which I think is wise. It is much easier to go from Kyrgyzstan to China to Pakistan.