Quick Slicks SC16 tires are designed specifically for the Scalextric Ford GT40 (stock) wheels.
Multiple silicone compounds are available - this is the X-FIRM compound (part number: SC16XF).
Dimensions listed below are with tire mounted on wheel.
Outer Diameter (O.D.): .819" (20.80mm)
Width (Contact Patch): .350" (8.90 mm)