Ahh.. I remember the good ol days of the Grapevine back in the late 70's when Mom & Dad piled us all in the old 1971 F250 with the 390 in it and towing a #6000 TT. We always got pulled down to 25 mph, foot to the floor. It even had 4.10 rear gears!
Anyway, I know times have changed.. Even that 2001 V10 should be able to do at least the 40 mph...
You'll have to get the revs up, so foot to the floor if you want the 55 mph you desire I would think...
When where the plugs changed last? What other maintenance done? It's only the 2V version of the 6.8, but they put those things in motorhomes too, so you won't just bomb up the grades "like it's not there", but it should still be better than the 5.3??
Anyway, too bad the manufactures don't offer a 3/4 SUV anymore.. Just think of a new "Excursion" with the 6.2 or the now 7.3 GAS engine and 6 speed or whatever trans they run in them now...
Since the pickup truck is off the table, your choices are pretty limited..
If the rest of the old Excursion is still in decent shape (interior, no rust, runs fine otherwise) Maybe get the V10 "refreshed" and maybe see what rear end it has and pop for the 4.30 if it's only a 3.73 now...
Good luck! Mitch
