Three pneumatic tires were used in a laboratory study to compare the performance of dual wheels at zero spacing with the performance of single wheels and to determine the influence of tire spacing on dual-wheel performance. One-pass tests were conducted on air-dry, medium dense to very dense sand. The data were analyzed for powered wheels in terms of pull, torque, and sinkage coefficients and of efficiency, all at 20 percent slip, and for towed wheels in terms of towed force coefficients. The existing WES performance prediction system was used in both cases. Results showed that a powered dual wheel at zero spacing, considered as two wheels sharing equal load and exhibiting equal performance, outperformed a single wheel with the same characteristics as each wheel of the dual-wheel configuration. The performance of powered dual wheels decreased with increasing wheel spacing until the two wheels performed like single wheels. This critical spacing was reached when it became two to three and one-half times the width of one of the wheels. A dual wheel at zero spacing, considered as one wheel, performed practically the same as a single wheel with the same characteristics of the dual-wheel configuration; however, the dual wheel with zero spacing performed more efficiently than the single wheel over a certain range of load, deflection, and soil strength combinations. (Author).