A vehicle suspension system comprising: a hydropneumatic strut comprising a fluid interface, where supply of hydraulic fluid to the strut via the fluid interface causes the overall length of the strut to increase, and withdrawal of hydraulic fluid via the fluid interface causes the overall length of the strut to decrease; a first displacement system in fluid communication with the fluid interface, capable of supplying and withdrawing fluid to and from the strut as well as measuring the volume of fluid supplied or withdrawn from the strut; a second displacement system in fluid communication with the fluid interface; and a hydraulic fluid source for selectively supplying or withdrawing hydraulic fluid from the hydropneumatic strut via either of the first or second displacement systems; wherein the first displacement system is in the form of a volumetric displacement system, comprising: a cylinder comprising a first fluid interface in fluid communication with the hydraulic fluid source, and a second fluid interface in fluid communication with the fluid interface of the strut; a displacement piston slidably retained within the cylinder and configured to move along the length of the cylinder between the first and second fluid interfaces (204, 205) in response to fluid displacement in the strut; and a sensor for detecting the position of the displacement piston within the cylinder; wherein withdrawal of fluid from the strut will cause the displacement piston to move toward the first fluid interface, and supply of fluid to the strut will cause the displacement piston to move toward the second fluid interface, such that the volume of fluid supplied to or withdrawn from the strut is able to be calculated according to the position of the displacement piston within the cylinder; and characterized in that the suspension system is further configured to selectively connect either of the first or second displacement systems to the hydraulic fluid source. a hydropneumatic strut comprising a fluid interface, where supply of hydraulic fluid to the strut via the fluid interface causes the overall length of the strut to increase, and withdrawal of hydraulic fluid via the fluid interface causes the overall length of the strut to decrease; a first displacement system in fluid communication with the fluid interface, capable of supplying and withdrawing fluid to and from the strut as well as measuring the volume of fluid supplied or withdrawn from the strut; a second displacement system in fluid communication with the fluid interface; and a hydraulic fluid source for selectively supplying or withdrawing hydraulic fluid from the hydropneumatic strut via either of the first or second displacement systems; wherein the first displacement system is in the form of a volumetric displacement system, comprising: a cylinder comprising a first fluid interface in fluid communication with the hydraulic fluid source, and a second fluid interface in fluid communication with the fluid interface of the strut; a displacement piston slidably retained within the cylinder and configured to move along the length of the cylinder between the first and second fluid interfaces (204, 205) in response to fluid displacement in the strut; and a sensor for detecting the position of the displacement piston within the cylinder; wherein the first displacement system is in the form of a volumetric displacement system, comprising: a cylinder comprising a first fluid interface in fluid communication with the hydraulic fluid source, and a second fluid interface in fluid communication with the fluid interface of the strut; a displacement piston slidably retained within the cylinder and configured to move along the length of the cylinder between the first and second fluid interfaces (204, 205) in response to fluid displacement in the strut; and a sensor for detecting the position of the displacement piston within the cylinder; wherein withdrawal of fluid from the strut will cause the displacement piston to move toward the first fluid interface, and supply of fluid to the strut will cause the displacement piston to move toward the second fluid interface, such that the volume of fluid supplied to or withdrawn from the strut is able to be calculated according to the position of the displacement piston within the cylinder; and characterized in that the suspension system is further configured to selectively connect either of the first or second displacement systems to the hydraulic fluid source.