I’m about to commence doing up an old horizontal dropout roadbike and think the best thing for it is to put it to a singlespeed, but I don’t know what size chainring / cog to go for? It’s pretty hilly around here, but is there benchmark (as per the 32:16 for mountain bikes) for road bikes?

Thanks in advance

Around 66″. Find the ratios on a gear ratio site like Sheldon Brown.

Make sure you have some brakes on it if not running fixed..!

48/18 is a good place to start.

or chainring x wheels size divided by sprocket = size in inches

42 x 27 / 18 = 63″

A 700c wheel is always 27

70″ (48/18) will be ok on the flat, but 66″-70″ will be more appropriate if there are hills.

all depends on how strong a climber you are. all imo 😀

I’m running 70″ (42-16) on mine at the mo which is fine for a flat commute (although a bit of a slog if a headwind is blowing!).

If it’s hilly maybe try 42-18 (62″)

Im running 40-16 on mine and find this a good compromise, still fairly speedy but can spin out going downhill, uphills are fine.

66″

44×17 with 700c wheels for me on my Roadrat. 69″ according to Sheldon

44×18 for me.

There’s a guy selling Stronglight singlespeed chainsets on eBay that I’ve got my eye on, and they’re either 40T or 46T, so based on going for the easier end of the advice above, should I be looking for either a 19T sprocket with the 46:

46 x 27 / 19T = 66.8″

or even a 20T (63.5″)

Does that sound sensible?

It’s got brakes and they’re staying on it! It’ll be freewheel for the moment, then maybe fixed later. Thanks for all the suggestions – really helpful.

if it’s not really hilly then 69-72 inches is good.
i run 72 in london and still do the odd ride around the box hill area with no problems climbing, it’s just descending that’s ‘interesting’ (fixed)

I use 70 -72″ in the Highlands without too much trouble. It will get me up anything a fully loaded lorry can climb on a main road. If it was flat I’d go to 80″, and if it was town use, I’d go for 60″ for acceleration.