(Delhi - Chandigarh ) - Very Good - One can touch about 140Km
(Chandigarh - Kullu ) - Till Anantpur Sahib, fine after that average Indian Roads, but still on the hills one can maintain average of 40-50Kmph
(Kullu - Manali) - The roads starts to worsen here. Still manageable
(Manali - Rohtang ) - The worst of all, especially the last 6-8 Kms.
My verdict: Give it a shot, you can always turn back from the lat few km, and have friends to help you out if in case you are stuck in Ice/Mud/
