So you say you're spinning away the winter miles inside, and you're a mileage-obsessed freak, but you don't have a cyclometer with a rear-wheel mount (or, shudder, rollers)? Is that what's troubling you, pardner?
Well, have I got a bodge for you: The Regular Front-Wheel Cyclometer Repurposed for Rear-Wheel Use, Albeit Out of Sight (or RFWCRRWUAOS):
Just slap a magnet on your rear wheel where it will pass closest to you left chainstay (probably right up near the rim), and then get creative installing the pickup magnet on the inside of the stay. In this case, my old Sigma mounts on a piece that's chunky enough to pick up a signal with no additional bodging, but I've tried others that required some kind of spacer under the magnet and some electrical tape artistry to work. The nice thing is, as an indoor fix, this doesn't have to be particularly durable, as it won't be seeing the vibration or weather that an outdoor fix would.
Next, the top end:
Obviously, since the head unit is on a short wire designed for front-wheel use, you can't get it all the way into the normal handlebar-mounted position, so you have to get a little creative. The old Sigma mounts with a rubber band, so it really lends itself to a lot of locations. I started with it on the top tube just ahead of the seat cluster, but my chubby thighs kept rubbing against it. Now, I've gone with the "guy behind you can see how fast you're going" location shown above. I can't see this while riding, obviously, but my whole intention is to just capture the miles on the bike so I can track service intervals (and yes, keep a vaguely accurate total for the year). As with the lower mounting, you don't have to be too tidy with your wire routing since this bodge will never see the real world (hence the sloppy, droopy wire that my OCD tendencies wouldn't otherwise tolerate). In the past, I've had good luck running the wire up the downtube and mounting it ahead of my bottle cage, too.
So there you have it: Ugly, but effective, like yours truly.