An elementary indicator of a car steerability, the understeering gradient, is determined on the basis of dependency of the steering wheel turn angle changes in the vehicle lateral acceleration function. So called comparative dynamic wheel turn angle is a component of an appropriate calculation formula, in which—for multi-axle vehicles—there exists so called equivalent wheelbase Le of a substitute twoaxle vehicle. This paper analyses methods for determining this parameter. There was used a substitute bicycle model of the vehicle with an equivalent wheelbase Le, to which a flat model of a multi-axle vehicle can be compared. There have been presented and compared two analytical methods of calculation that were illustrated with concrete examples of figures. Deliberations presented in this paper can be used to support calculations when analysing the steerability tests of three-axle and four-axle special vehicles, including the High Mobility Wheeled Platform (HMWP) – a universal purposes prototype vehicle.