'94-'97 OBS 7.3L
ZF 5-speed wide ratio:
1st = 5.72
2nd = 2.94
3rd = 1.61
4th = 1.00 DD
5th(OD) = 0.76
'99+ SD 7.3L
ZF 6-speed
1st(L) = 5.79
2nd(1) = 3.31
3rd(2) = 2.10
4th(3) = 1.31
5th(4) = 1.00 DD
6th(OD) = 0.72
I don't really think of it as adding a granny low the way that it is labeled on the shifter knob. Actually, when you look at the ratios, 1st in the ZF-5 is nearly the same ratio as 1st(L) in the ZF-6. Think about most peoples driving habits, at least mine anyway. I don't normally ever start in the actual 1st gear when empty unless I intend to go slow. If I'm driving a ZF-5 pickup I start in 2nd, and if I'm driving a ZF-6 pickup I start in 2nd("1" on the shifter). If I am loaded heavy I'll use the low 1st gear to start out. The way I look at it you gain another gear between 1st gear and DD, which makes the ratios in between 1st and 5th(DD) in the ZF-6 closer than in the ZF-5 leaving 1 OD gear on top of DD in both transmissions. I personally intend to someday do a ZF 6 swap into my '94 F350 at some point. It has 4.10 gears and a ZF-5 now. I would like to go to 3.55 gears and compensate with a ZF-6 to make up for the widening of the ratios by going to taller gears. I think it would be a great swap all around and shouldn't be to complicated on your 7.3L PSD. Mine is an IDI and is a more complicated swap.