Join Date: Dec 2004
Location: Middle of da Mitten
Bikes: Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, homebuilt recumbent
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Most recumbents that take a 24" wheel use either 507 or 520. My V-Rex24 came with 520, which I converted to 507 so I could use Specialized Fat Girl(tm) tires, which were subsequently discontinued.
You should be able to measure from bead to bead and it'd be pretty obvious which you had, since the ISO size is the bead diameter in millimeters (520mm = 20.47inches.) If by chance you have the 520 size, there is a 24x1 road tire available, the Panaracer Pasela. The 507 size is usually used in kids' bikes, often mountain bikes; so if you Google the size, mostly what comes up is knobby tires.
540, sometimes is referred to as "600A" and uses the same fractional sizing as 520, i.e. "24x1 3/8" which may lead to confusion. The ISO sizing should always differentiate them, though. AFAIK
there are no high-quality tires for it.