Re: 3 speed auto transmissions
 

Seems to me that when the 700-R4 came out,the instructions in the owner's manual stated that the consumer was not to tow in the overdrive range. Subsequently, nearly all if not all the automatic overdrive transmissions since then seem to all state the same thing. Driving back through a set of planetaries seems to create a reliability issue when under that much load.

Re: 3 speed auto transmissions
 

Greg, with the 700R4, it was a matter of a lack of clutch pack and band capacity, as well as a weak over run sprag. You do not see a lot of planetary failures with the 700 R4, or at least I have not seen more than a very few in the 20+ years I've worked on them. The over run sprag failed regularly, as well as the sun gear shell.

The problem with a single compound planetary would be size, weight, and complexity. I don't think there'd be enough of a reduction in rotating mass/weight by replacing two planetary sets with one larger more complex compound set. If there was a lot to be gained, the OEM's would already be doing it a lot, just for the fuel economy and emissions gains, as even a minuscule gain for them is big, spread over their unit production run.

Re: 3 speed auto transmissions
 

After 209,500 miles, the direct clutch pack in the 700 4R trans in my Chevy Silverado started slipping,flaring on the upshift, as it failed completely. The clutches were burnt up, the fluid stinks,this trans went the distance and owed me nothing. It's time for a rebuild. I did tow with it, used this truck for everything, no complaints. The 4L80 E in the shop truck at work came with a towing guide that specified not to tow in the overdrive position. It makes sense that that would put more stress on the direct clutches and the overdrive planetary set. Ford and Chrysler say basically the same thing about not towing in the overdrive range. That also prevents constant up and down shifting under varying road conditions under load which can't possibly aid longevity. Planetary gearsets are pretty reliable anyway,since they are in constant mesh. Heat and load seem to do in a trans more than anything else.Just ask a motorhome shop.