The wheel slip ratio is a function of the longitudinal speed of the wheel center, angular wheel speed, and Effective Tire Radius (ETR). The longitudinal speed of the wheel center and angular wheel speed can be calculated/estimated using an accelerometer and wheel speed sensor. However, ETR has uncertainties owing to variation in vehicle mass, air pressure in the tire, and dynamic load transfer. This article firstly demonstrates the sensitivity of ETR on wheel slip estimation using a Fixed Radius Recursive Least Squares (FR-RLS) estimator and analyzes its effects on Antilock Brake System (ABS) performance. Secondly, it proposes a Radius Searching RLS (RS-RLS) wheel slip estimator that mitigates the drift in estimates, maintains vehicle stability, and improves ABS performance. The proposed method was evaluated on a Hardware-in-Loop system with IPG TruckMaker. The RS-RLS estimator based ABS algorithm was compared against three FR-RLS estimator based ABS algorithms having different values of ETR and was found to have one order of magnitude lower mean absolute error.