For single speed there is a split in what you ought to do between fixed gear and free wheel single speeds.

For a fixed gear, unless you want to be spinning out when going down hill, you'd want between 70 and 80 gear inches.

For a freewheel, you can go lower and be alright. I'd say down into the 60's.

But, on to your question. A gear in a geared system will typically feel slightly higher than it will on a single speed drive train. I rode around in the jagged hills of West Virginia on a 46x15 freewheel, and found it absolutely perfect for climbing, descending and sprinting--of course, that was 82 gear inches.

On my former fixed gear bike I ran a 39x14, which on my wheels ended up as 75 gear inches. Just about perfect for not spinning out on the downhills out here in the Rockies.

But now I'm riding all geared bikes again.

Maybe that'll help.