Got a pompino with 48×18 which is pretty much spot on for road. What ratio would you recomend for a spot of flat offroad work, would a 48×22 cut it? Got a flip flop hub, want to get away with just swapping the wheel round but 18-22 difference I may not have enough space on the dropouts for chain tension.

may not have enough space on the dropouts for chain tension

Maybe you could try and do something clever with two powerlinks that allow you to shorten the chain by removing a short section.

I run a double ring up front and two different size sprockets out back, got a 42-16 and a 39-18, does the job nicely. I did it by using a white-industries double freewheel but you could achieve the same effect with a different freewheel on each side of a flip-flop hub. If you went that way you might want to offset the rear hub by 1/2mm to make sure the two different 'gears' had a straight chain line.

not a bad shout jack, dunno if they do power link type things in 1/8" will have a see.

I've had a powerlink type thingy on a 1/8" chain in the past – came with the chain though.

4 additional teeth difference will be an increase in the circumference of the sprocket of 2" (9" for the 18, 11" for the 22) which in turn means having to move the axle by approx half of that – eg 1". If you have 1" spare adjustment in the dropouts then you should be ok with the same chain length.

I have used the same chain length on a 49:20 as a 49:16 on my pompino.

never tried off road, but have toured at 67" (49:20) and now generally ride 49:18 on road.

Of road 29er MTB is something like 52" (as you would for 26er off road)

63XC is the fixed off road bunch/blog/forum thing and take their name from 63" as the preferred gearing for fixed, apprently. Ask richpips