A few years ago I found that I have a "measured mile" on a road near my house. I use the bike computer's documentation as a guide to get the measurement close. I then take a ride with the appropriate tire pressure and then I ride the measured mile (there is also a sign along the way for the 1 KM mark). Last time I was within 2%. I noted setting that was used on my biek computer and adjusted it 2%. Next time I rode by I was within 1%... good enough for me. There is quite a difference between tires that are listed as the same size, but of different manufacturers or different models.

I tried the measure on the floor thing, but forgot to add my weight to the bike so I was off about 10% the first time I did that. It does work, but it was a bit of a pain.

Happy riding,
André