Planetary rovers play a significant role in lunar and Martian surface explorations. However, because of wheel slippage, the wheels of planetary rovers can get stuck in loose soil, causing the exploration mission to fail. To avoid slippage and increase the drawbar pull, the wheels of planetary rovers typically have lugs (i.e., grousers) on their surface. Recent studies report that these lugs can substantially improve the travelling performances of planetary rovers. Therefore, in this study, we conducted experiments using lightweight two-wheeled rover testbeds in a sandbox to provide a quantitative confirmation of the influence of lugs on the travelling performances of planetary rovers. Based on our experimental results, we confirmed that although lugs have some effect on the travelling performances over gentle slopes, they have a greater effect on the travelling performances over steep slopes.