We routinely cross the continental divide with our boat into Montana, and also traverse countless unnamed but far steeper grades. Look at a map of the US and put a dart in the northernmost part of Idaho and you'll see the epicenter of our boating territory. We go to British Columbia, the Puget Sound, Montana, local lakes and the 80 has been terrific. Don't get me wrong, it's not even in the same league as the dedicated tow powertrains that began appearing about 12 years ago (Powerstroke diesels, Cummins Dodges, etc) but we do fine. It's important for you to know that I'm not the kind of guy that's bothered by doing the last 2 miles of the worst passes at 25mph with the flashers on while other travelers whiz by at 70, so my version of "fine" may drive some of you nuts.
The very worst pass I've ever been on is east of Wenatchee on Hwy 2. That thing is unbelievable. I'd have to guess that my foot is within 1/4 inch of all the way floored for at least 5 minutes in first gear at 20mph. I've done it on a 95 degree day with the air on and seen the temp needle move a needle width. Just got my oil analysis back from Blackstone and all is well with my engine internals despite this severe use.
As someone else said on this board a while back, there are certainly better tow vehicles out there, but I would not consider walking away from the 80s incredibly well balanced attributes and reliability if it can get the job done in the extremes - even if it's not the first over the pass. Ya gotta give that title to the big domestic iron.
On the flats, we cruise (literally with the cruise on) at 70mph unless there's a strong headwind (boat is lower, taller and wider than the 80 and punches a HUGE hole in the air), though certainly traffic considerations dictate this. I know many of you from back east may think that's unsafe, but out west it's not uncommon to be the only vehicle in site on an interstate. I have also done 2 memorable panic stops with the boat and it's not pretty. Also, one emergency avoidance maneuver. I would definitely NOT tow like this without rear airbags to control the load, and the usual anally retentive maintenance I do.
On speed, I noticed quite a difference when I advanced the engine timing to 7 degrees. Some difference in everyday driving, but particularly in the extreme parts of towing over passes. Obviously premium gas and top grade lubes are even more important if you do this mod.
DougM