I have foolishly agreed to partake in a duathlon in February. The cycling part is a 20km flat tarmac route. Obviously a road bike would best, but I don’t have one and can’t afford to buy one either.

Now I won’t be taking this too seriously, so I thought I would just make some changes to my singlespeed and use that. I was thinking slick tyres and a change of gear ratio. I currently run 32/16, what would be the best ratio to try on road?

If its flat put a 12 on the back.

There’s a good table of gear inches for various chainring/cog combos on Charlie’s site, halfway down this page: http://charliethebikemonger.com/page47.htm
FWIW I use 36/13 on my commuter (mixture of road and towpath). If it’s properly flat I’d go bigger than the 12 petesgaff recommends – maybe stick a 44 or 46 on the front?
EDIT: Whilst keeping your 16 on the back.

44×16 or, if you’re feeling fitter, 48×16.

Remember if you are planning to switch tyres too, such as Conti Sport Contact 1.3s for example, this will effect the gearing too.

I’ve an unramped 38t ring for sale if thats any good.
That should give it longer legs.
A 44:16 might be a bit much.

I’ve been running my SS with two cogs at the back, as it has to double as a cummuter sometimes. Just enough dropout length to switch between 15 & 17. The 15 is still too spinny on the road so I have some spare slick tyred wheels now. Planning on going back to a double at the front (no shifter though), & running it 36:13 on the road/32:17 off-road. Theoretically I can keep the same chain length then.

32×11 on 1.25″ slicks (narrower tyres are also less high and thus lower gearing as well) would give you a gear inch of 72″ish…

72″ is a decent gear to spin along the flat with.

useful site to work out ratios inc changes due to tyres

http://www.sheldonbrown.com/gears/