Struggling up hills.....Can I change gearing ratio etc. to make it easier ??

[bpsmith]

Current 105 is 11speed. For 10 speed you need current Tiagra, but shops may stock old 10spd 105 cassette.
There is no difference between 10spd 105 and 10spd Tiagra cassettes, I imagine the tooling is identical, they just rename.
SRAM 10spd cassettes work just as well.

The material used and the way the the cogs are fitted together is completely different between Tiagra and 105, along with tolerances being tighter on 105 also. This is the technical answer. The real world difference is something else, I hasten to add.

 

[MichaelW2]

Yes, 105 uses EV technology which is an aluminium carrier for the larger sprockets.
My large Tiagra 9spd sprockets are quite flexy, which is not good. Thinner sprockets are designed for top end groupsets using the carrier to enhance stiffness. As the cog-count trickles down the groupsets, at a certain point they use simpler construction, better suited to sprockets made from thicker steel.