An adjustable width chuck assembly for a tire testing machine including upper and lower relatively movable rims by which a tire is clamped and held during a testing cycle. A pilot or nose cone forming part of one of the rims is gas pressure biased towards engagement with complementally formed structure on the other rim. The gas pressure bias is provided by a gas spring which can be replaced with gas springs of differing pressures in order to adjust the biasing force or, alternately, the gas spring can be removed from the chuck assembly and re-pressurized to a different level in order to change its biasing force. The use of a gas spring for providing the necessary biasing force expands the range of motion for the nose cone, thus allowing a given chuck assembly to accommodate tires having a wide range of bead widths.