Why can my rear derailer no longer shift between all 8 gears? After a low-speed collision with a car. my LBS replaced my damaged rear derailer. Shortly after, I replaced chain and cassette (from a SRAM bundle) and the front chain ring myself, as I've done several times in the past.

After adjusting the derailer while shifted to the 8th and highest gear, so the pulley wheel aligned with the smallest cog, the shifting was smooth all the way down to the 3rd gear. But I was not able to reach 1st or 2nd.

Adjusting the derailer while shifted to the lowest gear, shifting was smooth from 1st to 6th, but I was not able to reach 7th or 8th gear.

My derailer seems to span only 6 cogs.

So far I've tried (without luck):


Cleaned and lubed derailer hinges
Losened both limit screws significantly - to the point where I can push the derailer past smallest/largest cog, without using excessive force
Removed the chain and shifted back and forth, so the derailer can move freely - still only spans 6 cogs when using shifter
Took the bike to (another) LBS who adjusted the cable to it kind-of spanned all 8 cogs, but didn't run properly, let alone smoothly, in any of the gears - worse off than with 6 well-working gears!


What could be wrong? Gear cable? Cable housing? Shifter? - Any help greatly appreciated! :)
It sounds like you may have bent your deraileur hanger.

Looking forward from the rear of the bike, the hanger should be perpendicular to the ground when the bicycle is standing straight up.  

Some bikes have replaceable hangers.... For others there are tools for aligning bent derailleur hangers....  Absent the tool and depending on your comfort/mechanical skill level it is possible to align the derailleur hanger with a large adjustable wrench by removing the derailleur and rear wheel and applying gentle force to return it to its perpendicular position.  

If it is bent to severely, I highly recommend taking it to a shop.