An activating portion (70) on one section (36) of an adjustable wheel assembly (26) triggers a proximity switch on a vehicle such as a field sprayer to provide a signal indicative of wheel tread. The signal is sent to a controller (50) which controls wheel tread adjustment. Two or more assemblies (26) may be adjusted simultaneously reduce adjustment time. The operator control (60) is located in the vehicle cab (22) for entering wheel spacings, providing an indication of the actual spacing and individual wheel position, and providing error and warning messages. An interlock system prevents axle telescoping unless certain vehicle conditions are satisfied, such as the vehicle speed being below a preselected point and vehicle being in the field mode.