There are 4.. 349 is a High pressure performance 16". 305 , rim smaller tire wider ..
451 and 406 are the 2 in 20".. 406 is the most common as its the BMX bike size..
A 349 - 32 (Schwabe Kojack) at 100+ psi , if its slow the rider is just slacking.
Measured difference is the circumference, so for a measured mile the number of times a 20" wheel
will rotate, vs A 16" wheel can be calculated..
./.