The International Roughness Index (IRI) is a key indicator for evaluating the performance of road surfaces. However, traditional measurement methods only focus on the evaluation data of a single longitudinal section and do not consider the lateral difference between the actual contact area between the tire and the road surface, which may lead to inaccurate evaluation results. In recent years, with the advancement of 3D laser scanning and digital photogrammetry technology, full-section data acquisition has brought new possibilities for roughness evaluation. However, how to find a balance between data fineness and computing efficiency has become a core problem that needs to be solved. Based on the principle of interaction between vehicles and road surfaces, this paper proposes to include only the pavement height data within the tire width range into IRI analysis, and establishes an evaluation framework based on standard tire-ground contact width. This method not only retains the key information of horizontal unevenness, but also increases the computing speed by reducing the amount of data. In the experiment, a vehicle-mounted three-dimensional measurement system was used to measure the height of the actual roads, and the variation patterns of IRI values under different contact widths were analyzed. The study found that the tire-ground contact width will affect the IRI calculation results, but the degree of impact varies according to the wheel position. The IRI value of the left wheel area is not sensitive to the change in contact width, while the IRI value will fluctuate significantly due to the lateral slope of the road surface or structural asymmetry in the right wheel area. We recommend that 200 mm be used as the recommended value for the tire-ground contact width, which can not only ensure the evaluation accuracy, but also improve detection efficiency.