The wheel slip ratio is an important state variable in terramechanics research and the control of planetary rovers. Definitions of the slip ratio for a wheel with lugs and methods of estimating it for all wheels onboard have seldom been attempted. This paper presents several definitions for the slip ratio of a lugged wheel, which can be interconverted by altering the shearing radius. Equations for calculating the longitudinal velocity and slip ratio of a wheel moving on rough terrain are deduced from the horizontal speed of the wheel's axle. Wheel-soil interaction experiments were performed for two types of wheels with different radii and lugs of different heights. The drawbar pull, torque, and wheel sinkage were measured using sensors. These data confirmed the effectiveness of the proposed slip ratio definition methods. Furthermore, two slip ratio estimation methods are proposed and verified: a visual information-based method by analyzing the lug traces marked on the terrain with high precision, and a terramechanics-based method in which the equations for the vertical load and torque are solved to estimate the slip ratios of all wheels.