I didn't mean to give a 'scare' factor to the post. The speed would be comparable to other sessions on the same spin bike, and you could 'manufacture' the number you input to the speed sensor to give something you feel is realistic. For example; the first time I got my speed sensor for my Garmin was a GSC-10, I couldn't be bothered to measure my wheel size and I couldn't ride it to calibrate it (weather, time). So I just did a session from 3LC. According to the sensor, I was warming up at 60mph and during 5 minute efforts I was averaging speeds of 150mph. I think you could expect similar recordings for your spin bike (i.e wildly unaccurate speeds, but they're comparable if you use the same spin bike).

Personally, I think you'd be better off measuring HR/Cadence/Time...