Abstract: Apparatus for seating and inflating tubeless tires, particularly tubeless tires of large sizes, wherein a wheel having a tubeless tire placed thereon is elevated into engagement with an annular wheel rim seal ring and a tire sidewall seal ring and held by a predetermined force. The sidewall seal ring maintains a separation between the upper tire bead and upper wheel rim, and an annular chamber defined between the seal rings is pressurized with air permitting air to enter the tubeless tire between the upper tire bead and wheel. The pressures exerted upon the sidewall seal ring during inflation are sufficient to lower the wheel and tire permitting seating of the tire bead and wheel rim. Lost motion means support the wheel rim seal ring so that sealing thereof with the wheel rim is maintained during lowering of the wheel and tire under inflation forces, and the seal rings are mounted for vertical adjustment to permit ready accommodation of the apparatus to various sizes of wheels and tires.