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when you think that your going fast enough towing that OD is safe, think again. the cluches are only designed to hold so much before they slip. when you are towing something in OD its on the edge or over. now when you go faster your are adding more HP so now its further over the edge. but going slower lugs the motor and you step on it to move and it slips again.
now for the educational part.
with the gear selector in OD, and the tranny in 4th, there are only 3 "contact" points holding the tranny. the 2-4 band, the forward clutch, and the 3-4 clutch. there are 6 clutches in the forward clutch and 5 in a stock 3-4 clutch(9 in a upgraded clutch, not every rebuilder puts these in).
with the gear selector in OD, in 3th, there are also 3 holding points. the forward clutch, 3-4 clutch, and the forward sprag. the sprag will hold alot as long as its not worn severly.
with the gear selector in D, in 3rd there are 4 holding points. the same as 3rd in OD, but now you add the overrun clutch. it only has 3 to 4 clutches but that 3 to 4 will add alot of weight/hp to the holding power before it slips.
just because its not hunting for a gear doesn't mean its not wearing on the tranny. when ever i pull apart a tranny i can tell if they towed in OD or not. the wear on the OD parts is abundant compared to standard driving in OD. more wear means less the tranny lasts.