I've been practicing treating my bike as a single speed for a bout a week (100mi+ without changing gears). I have an '07 Trek Soho (link) with the stock gearing setup 42 x 11-32 (9spd). I've been riding in 42 x 18 (2.33ratio) with slick tires and found that at a comfortable effort and cadence, I'm rolling 17-18mph flat, 13-20mph on hills.
My plan for the winter is to convert to a single speed with (eccentric BB so it will be an easy convert) a studded front and cyclocross rear. While I know that at the same cadence, my speeds will be the same with a 42 x 18 setup, but I'm wondering if the extra weight and grip of the new tires will cause significantly more effort.
Should I go with a 42 x 19 instead for slower speed or just work a little harder on the 42 x 18 until my muscles make the extra work feel like it's normal?
First off, I would recommend studded front and back. There is no shame in that. Don't let the tough guys fool you, You'll get better traction and be safer with both. Plus if you are riding through the winter you ARE one of the tough guys.
Secondly the studded tires are harder to roll for sure. I up my gearing on the SS for winter to make up for the lower tire pressure and much higher drag from the studs and tread. Not to mention the extra layers of clothing and dense cold air. It's harder in the cold period. I ride a 42-16 in summer on 25 mm slicks and 39-17 in winter on 35mm studded.
A single speed drive train is more efficient with the straight chain line and no derailleur to go through so that's helpful. But make no mistake, single speeds in the winter with studded tires will make you work, and make your legs strong.