Magic of recovery? Broken computer? Something else?
 

Weather has been a little gloomy recently, and I've been feeling I've neglected training, so I decided to hop on the rollers for a short, perfunctory spin. Cadence and speed started out slow, as usual (low 70s, 14-16 mph), but after about 15 minutes, I managed to get it up in the 90-100 range, and since it didn't feel like any effort, I put it up on the big ring and started dropping cogs. Well, I managed to spin it up, and was quite surprised to see it registering in the 30s, and at one point sustained a solid 36mph for about half a minute. I was rather incredulous, because normally, on a stationary bike (the only time I really pay attention to these things), I'm lucky to get it to stay in the mid 20s and peak at maybe 30 momentarily. Why can't I do that outside? I wondered dumbly. After catching my breath, I spun it up again, and managed to get it to hold 40-41mph for about half a minute. WTH? Did I break my computer? Seriously - those numbers are completely anomalous; but it's true, I was on 53/12, spinning my legs like crazy.
Sure, lack of wind resistance would make a big difference between what can be done on rollers and what can be done outside, but that doesn't explain why those numbers were so much higher that what I've ever experienced on a stationary bike. Who else has had this experience, and to what would you account such a leap? Proper fit/positioning, maybe?

Speed info on an indoor bike is not reliable.

Sorry, but that's the truth.

ps in your case it's probably because of lack of air resistance. You could probably go that fast on the moon or in a vacuum. Not IRL.

Lol. What datlas said.

The resistance of the rollers is likely not the same as the resistance of riding on actual roads as well, as well as the lack of wind resistance, inclines etc.

OP description is a little confusing because it compares speed on the rollers to speed on a "stationary bike". Unless "stationary bike" actually means riding the same bike on the same rollers, any comparison is pretty meaningless.

Now, if it is an apples-to-apples comparison, then the improvement may be attributable to the effects of a "taper". Modern approaches to training model stress (acute training load) and fitness (chronic training load) using exponential decay formulas. Training stress balance is the difference between your current stress and fitness. Since stress decays much faster than fitness, extended recovery will increase your training stress balance resulting in a (possibly significant) performance improvement.

Of course without detailed tracking of all your training, all we can do is speculate. But for someone who has a pretty consistent training load week after week (with no periodization), what you observed is pretty typical.

This is where I figured it was going. Since indicated speeds on stationary bike at gym were more consistent with on-the-road (i.e., hard efforts and sprints hovering in the 20s, peaking just over 30 maybe in both cases, it seems the bad fit and position takes away any gains enjoyed by a lack of wind resistance, whereas on well-fitting bike on rollers, nothing takes away from the advantage gained. Still, it's hard to believe fit and position could make the difference between being able to "sprint" inside at 40 vs 30 mph.

On a stationary bike, by definition you're going 0 mph, just saying...

I can hold 35mph on the CycleOps trainer for an hour, with the resistance set to 6/10. There's no wind resistance, no rider weight, no bike weight, no irregularities of the road surface, etc. I'm just on the trainer when the weather is awful, and I want to keep my heart rate in a specific zone. The speed and distance numbers are essentially meaningless.

On rollers your resistance is nowhere near what you see on a flat stretch of road unless you a) have a resistance unit fitted to the rollers or b) take your tires down to about 45-50 psi. I noticed when I deflated the tires a bit, speeds were still high, but much more realistic for something like a flat straight for a given HR.

Thanks for the insight - it's starting to make sense. This morning it was fully inflated 23mm tires, and come to think of it, a while back, trying it out with soft 25mm tires on the FG was the most difficult. The rollers are 3" drums, don't have a resistance unit, but they are more difficult to start from a dead stop than I would have expected. As for the stationary bike at the gym, I tried to set the resistance high, but when doing strength/intervals on it, I'm not sure it works - it's like there's zero resistance until it's time to put forth the effort. Maybe it's supposed to be like that, but I like to have SOME resistance while I'm resting, and I feel like I do on the rollers. I guess the stationary bike just does a better job at providing/maintaining resistance at higher speeds.