This is the Primo V-Trak, 24" x 1-3/8", 37-540 High Performance, Knobby tread, pneumatic tire for wheelchairs. This tire has a C763 tread pattern and the maximum air pressure is 65 PSI. These are gray non-marking with black walls. This tire requires an inner tube, sold separately. Try our standard inner tube (see part #I047P) or our thorn resistant innertube (see part #I121P). Non-marking means that these tires will not leave skidmarks.