That fish must have been 6 feet long!
OK, call me a curmudgeon–a slow curmudgeon.
My fastest time on a 36" is about 16.7 mph according to my cycling computer. I am sure I could go a bit faster. Since most unicycles are fixed gears, we can convert the speed into rpm using only a few simple assumptions:
Many of these speeds are on 36" unicycles. A typical measured diameter of a 36" unicycle under load is just under 280 cm. I will use this value.
At 1.000 MPH, the number of cm traveled in one hour is 1.609 X 10^5. Dividing by the wheel diameter in cm gives me the number of rotations in one hour: 574.6 rotations per hour. Dividing by 60 minutes in an hour give the rotations per minute = 9.58 rpm.
On a fixed gear 36", each one mile per hour corresponds to 9.58 rpm. To get the total rpm, just multiply this constant (9.58) by the mph.
As an example, let’s take the value of 22.9 MPH reported by Scott Wilton, who is one of the fastest unicyclist around (he won the criterium in this years Nationals). His 22.9 RPM would correspond to 219 rpm. This value is far above the cadence cyclist usually ride. However, the speed sound credible.
As a second example, let’s try some old codger like myself. 
My 16.7 rpm would correspond to 160 rpm. Not very impressive, but certainly doable.
We can also examine some of the Schlumpf hub records. In this case we need to divide by about 1.5 ( I am unsure of the precise gear ratio.) If we take Corbin as an example, his 26.1 MPH would be reduced to 17.4 mph with the Schlumpf correction, and the cadence would be 167 RPM–that is almost an old man’s cadence! Admittedly, 26.1 mph would scare the crap out of me, but I do not have a Schlumpf hub.
Now let’s examine two of the most recent claims (but there are many others I could pick on.) Madison Johnston reports 28.0 MPH, which coverts to 268 rpm. That is a crazy high number, but he does report it going downhill.
Still, for many of these claims I would like to see the speed independently clocked by some more reliable method than our cycling computer. Alternatively, why don’t we report what circumference we are using in our cycling computers. If I put in 380 cm my speeds are MUCH higher.
Finally, we have a report of 40 mph on a (fixed?) 29". Using the same method, with a circumference of 231 cm, I find an cadence of 464 rpm.
Hmmm.
Orange County Skeptic