OK, I am basing this solely on the cheap Taiwanese speedometer that I got from Spooky Tooth. If you check the picture below you will see that 200 rpm equals 21 mph so I'll start from there:

One revolution is equal to the circumference of the wheel and the circumference of a circle is equal to pi times the diameter so the diameter is equal to the circumference divided by pi:

diameter = (110.88 in) / (3.14159)
= 35.29 inches

So unless you have a mighty large front wheel these speedometers are useless! Likely they are a kids toy designed to make them think they are going a lot faster than they actually are.

thats assuming the rotations per minute are accurate.

does anyone have a correction factor for the analog taiwan speedo that is advertised as suitable with 26"/27" wheels when using the 26 inch wheels? lets say according to the instrument im going 60km/h how fast am i really going?

I use 191 for the digital for both 26" and 27" which are actually same size. Dead on compared with GPS.

These are very reliable once you take care of the reset problem and incredibly easy to set up. Only takes a few seconds. Calibration is also trivial assuming you have a cheap GPS (who doesn't these days?).

I won't put my money on it because I only just ordered it, but for those who want a classic look, why not go to the whizzer motorbikes catalog and buy one of their mechanical speedos. As opposed to those really cheap ones on ebay and spooky tooth, this speedo is made for the 26" whizzer wheel and I have never read any complaints about them. I am excited to get mine! After shipping it came to about $33. Just a suggestion

I hear the Wizzer speedometers are a viable option, that they're at least designed for these speeds... but I dunno about calibration, accuracy & wheel/tire sizes *shrug*