I have lots of power at higher rpm. Reving 3000 to 3500rpm is where it wants to pull in 4th. Its in lower rpm range its got nothing. If i put it into 5th i lose speed and cannot maintain.

 

Perhaps the -25C temperatures we have been having lately is causing significant drag in the drivetrain? Don't know but I have never towed anything in these temperatures.

 

I've towed in similar situations (last winter in Alberta/Saskatchewan) with my EB maxtow and my old '12 GMC 4.8. Both trucks had an easy time pulling my 5k dry/ 6100# loaded TT all along the foothills of Southern AB and the prairies of mid-southern Sask. This baffles me. It almost seems like the bearings on the trailer axles are unable to efficiently travel at speed. I remember using an exceedingly old cargo trailer from my dad a couple years ago and it wasn't rated for over 88km/h, and it pulled like **** at anything higher.

Other than that theory (and I have my doubts it's related, but still possible), I got nothing.

 

Its brand new trailer and everything seems good i have jacked trailer up and made sure the bearings are running free as they should on a new trailer. I had same troubles with 17' camper trailer that was 3500# this summer so i am assuming the is a problem with truck not trailer. Just to try it for giggles i my sled trailer to my buddies 4.7L Ram and his truck never knew it was there. Again this would suggest an issue with the truck to me not the trailer.

 

Not sure either but since i had same issues with my 3500# camper this summer in +30 temps im gonna go on a limb and say its not the trailer or temps.

 

Once you are on the road for a few miles everything is warm and should tow easily. My2012 5.0 354 gears tows a 7000 lb holiday trailer in summer, at 110 and gets about 14 miles per gallon, level with no wind.

On large hills it will wind up to 5500 rpm with cruise control on.

The plugged exhaust idea is worth thinking about. I had rental car on muddy roads years ago, at low speed it threw mud in the the tailpipe where it baked on. I got down to 30 mph and shifting down did not let me speed up.

I stopped, put in neutral and tramped on it with the windows open.
A great whooshing noise, but it could not wind up with no load. I beat on the end of the tailpipe with the wheel wrench, knocked the mud free and away we went.

I've seen a Dodge van that was using leaded fuel and l plugged the cat, with the same symptoms.

So test this. Rev it up in neutral. Can you get to 5000 rpm?

If you are doing 80 in 4th, what happens is you shift to 2nd? Slow down? Can you hear the exhaust?

 

This is a real brain tickler, there is no reason your truck should have any issues pulling that, heck it should barely notice.

 

Is it safe to shift to second while driving 80?

 

At 100 kph (62 mph) towing on cruise control, mine will shift automatically to 2nd and run up to 5500 rpm. I don't mind it for short while, a minute of two, but I know roads where the climb might be 5 miles, at which time I will make it manual and use 3rd.

If it was not safe, Ford would not have made it to do that.
Ford claims the 5.0 motor max power comes at 6200.

2nd gear at 80 km/hr is not an issue. Just tramp on it from a dead stop and hold the loud pedal down to see.

 

Yeah from dead stop to 80kph no problem. I do believe it will stay in 2nd or just into 3rd as I hit 80kph. I haven't tried the rev to 5000 rpm yet.