So what do we mean by tread width? The early FC-150 had a narrow tread, while the late FC-150 had a wide tread. The FC-170 had a wider tread still. Below is a pictorial attempt to show, head-on, the different Forward Control tread widths.

EARLY FC-150 Initial production through 65548-18205.	LATE FC-150 65548-18206 (July 3, 1958) to end of production.	FC-170
|----48-7/16"----| Spicer Model 25 front axle. Capacity of 2000 pounds.	|--------57"--------| Spicer Model 44-IF front axle. Capacity of 3000 pounds.	|-------63-7/16"------| Spicer Model 44-F front axle Capacity of 3500 pounds.

Note that rear axle tread width equals front axle tread width except in the case of the FC-170 DRW which has a rear axle tread width of 72-1/4".

The change to a wider tread in the FC-150 was due in part to a change in suspension design as well as a new axle. On the left is the early FC-150 suspension. Note that the axle mounts above the springs and that this is essentially identical to the CJ-5 front suspension. On the right is the late FC-150 axle. The springs mount to outriggers and the axle is attached below the springs.