The split ratio of rotor outer diameter to stator outer diameter is one of the most important design parameters due to its significant impact on machine torque density and efficiency. It has been optimised analytically in existing papers with due account given only for the thermal limitations. Here, the optimised split ratio for high-speed permanent magnet machines (HSPMM) with consideration of the mechanical constraints is investigated analytically. In addition, the influences of flux density ratio, maximum operating speed and sleeve material on the optimised split ratio are discussed in detail. The analytical results are verified by finite-element analyses. It is demonstrated that the optimised split ratio as well as the achievable torque of HSPMM is significantly reduced when the mechanical constraints are taken into consideration. The experimental results on a 6/slot-4pole HSPMM confirm the validity of previous analyses.