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A 21 inch wheel is a Porsche, Tesla Model S and X, Audi and large sedan size, almost always staggered and almost always a forged or flow formed design. The sidewall is 30 series or lower, so the tire options are luxury summer lines. Going up a diameter trades sidewall for wheel. The car steers a little sharper and the ride gets firmer, and the caliper sets the floor.
213 designs on this site come in 21 inch, in 8 widths from 9 to 13 inches wide. 21x12 has the most designs (159), then 21x10.5 (94). The bolt patterns with the most 21 inch designs are 5x120, 5x112 and 5x114.3. A 21 inch wheel clears or rubs on its width and offset, not its diameter, so pick your car and the page keeps only the sets that clear, flush or tucked as you choose.