Switching from a 21speed to 2speed setup

I want to shed some weight off my trusty mtb and was thinking about starting with the crank and rear cassette..
my question is : is it possible to have a 21speed MTB reduced to a 2speed? My reason is because I really only alternate between 2 rear cogs when I ride on+off road, so the rest are pretty useless for me!!

Just go singlespeed instead.

Singlespeed conversion kit (using the cog from the cassette) + removed big and small ring = bob your uncle.

You're going to go to a single chainring and have two cogs at the back?

If it were possible (seems unlikely you'd be able to adjust the H / L stops on the mech to such a tight range as two cogs, but it might be doable) you'd need some kind of conversion kit as ed suggests to pack up the space on the freehub that the cassette currently takes up, so the only weight you'd save would be the difference between the conversion kit weight and the bits of the cassette you don't want. You'd still need a shifter and a mech.

It doesn't sound like you'd save much weight given the hassle, so I'd say either go singlespeed, of keep a full cassette at the back.

I reckon you could run two sprockets on the back with a 7-speed spacer (3mm ish) and then swap it over manually (ie. loosen wheel, move chain).

What ed says, but what ratio? I've run 32x16 which is quite capable off road though top speed is limited, 34x16 still useable and don't go higher than 36x16 or you'll suffer. These are just ratios you can achieve the same gearing with other numbers. Find a magic gear computer online to suit your chainstay length and avoid using a tensioner if possible too.

this is wayyy to much hassle! lol.
i think ill just do away with the whole gearing thing as Ed says and try some ratios like Alkali suggests. thanks guys for all the advice!

chain tensioner will be fine.

chain tensioner will be fine.

thanks ed - why did Alkali say not to use one...is it because of the extra weight coz my Gusset Tensioner is only a few hundred grams if that...

no, just it looks nice if you can get the right ratio. I used a tensioner, no probs with it. What frame you got?

its a Kona Dew with a rigid fork

chain tensioner can look ugly, but it's inevitable, they work pretty well if you think about derailleur being more vulnerable than chain tensioner.