Trucking Efficiency recommends 4x2s for duty cycles that allow for the lower load capacity.
For duty cycles with lower loads, a single rear axle may improve efficiency by removing one axle and lowering tire rolling resistance. Commonly called single-axle tractors, 4x2 tractors lower fuel consumption by having one less axle and one less set of tires. These modifications reduce friction and tire rolling resistance on the road surface, thereby reducing fuel use. 4x2 tractors are less costly, but are limited to routes with lighter weight freight.
We are not planning a report on 4x2 axles, but may consider them for future evaluation. For more information, contact your tractor builder.