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Message: [QUOTE="me2, post: 384471, member: 3825"] Here is how towing works with the 6.4/5R110. I suspect it would be worse with a 6.0L. Here is the power curve for a 6.4L engine. [ATTACH=full]91133[/ATTACH] The 5R110 ratios are 0.71:1, 1:1 and 1.54:1 On a flat highway with no headwind, my F350 pulls my trailer quite nicely in OD. No complaints there. The engine purrs along, everything is quiet and smooth. Engine speed at 60 MPH in OD, with 20s is 605 revs/mile x 3.73 x 0.71 = 1600 RPM. At 64 MPH its 1709 RPM. Peak torque on the 6.4L is 650 ftlbs, but that is at 2000 RPM. At 1600-1700 RPM its making about 550 ftlbs. 1600 RPM x 550 ftlbs / 5252 = 167 HP. 1700 RPM x 575 ftlbs / 5252 = 186 HP. So it takes way less than 200 HP to pull my trailer at 60+ MPH in good conditions. And that is assuming that the transmission and engine controller allow the engine to max out its power before downshifting. I'm not so sure that is the case. FWIW, my new trailer is a lot more aerodynamic than my old one was. I suspect that on flat ground the two needed about the same power. Things start to get interesting when you add hills and/or headwinds. At some point the transmission and engine decide its time for a downshift and it does take much of either to make it happen, especially when its a hill. When I say hills, I am not talking about mountain passes. I am talking about hills in otherwise flat country. Hills on a secondary 2 lane highway where there is no passing lane. Those sorts of hills. Engine RPM in direct at 60 MPH with 20s is 605 x 3.73 = 2250, which is not the end of the world. At 64 MPH, its 2400 RPM, which is liveable if its not all the time. At 2250, the engine is making 2250 x 640 / 5252 = 274 HP. At 2400, the engine is making 2400 x 630 / 5252 = 287 HP. This is assuming that the engine and transmission controller allow the engine to make full torque. Once again, this is all pretty much OK, provided that the truck doesn't need to run in direct pretty much all the time, which sometimes happens in a strong headwinds and hilly country. Then life is a continuous series of downshifts to get up to speed and downshifts when the truck can't maintain the load in OD. The real problem happens when the truck decides it needs more power and downshifts into 3rd. Engine speed at 60 MPH is 605 x 3.73 x 1.54 = 3475 RPM. Actually its more like 31-3200 RPM, for various reasons I won't get into. The engine is making about 330 HP here, only about 40 HP more than it was in direct. But now its turning 50% more RPM. So there you sit, engine screaming away going up the hill. Sooner or later the cooling fan kicks in and now you are down to 20 HP more than you had in direct at 2400. As far as I know there is no way to lock the 5R110 out of shifting into 3rd gear. As a matter of fact, the shift happens sooner if you are using Tow Haul mode. The only way to control the transmission is to turn off cruise control, if you were using it, and manually feather the throttle to keep the transmission in direct instead of 3rd. That means running it at about 75% of full power, which is about 275 x 75% = 206 HP. Which is only 20 HP more than we had in OD ! And by this time you are down to about 45-50 MPH. If you are on a 2 lane road and there is traffic behind you, they are not happy. The only other choice is to floor it and allow it to downshift back into 3rd and scream away. I find this all EXTREMELY frustrating. There are other issues too, like the downshift points when engine braking is needed and how the TC never seems to be locked when accelerating, but I'll leave them out for now. Stock for stock, the 6R140 is 3 times the transmission the 5R110 is because a) its got more/better ratios, b) its stronger, c) its got a much better TC, that allows way more engine lugging and d) it allows 2 modes of manual control in addition to better automatic control. The more I write the more frustrated I am that I don't have my swap done. I'll be towing our trailer about 3,000 miles over several mountain ranges in August. I'm writing all this from a vet's office. Our dog somehow got sick during our travels. [/QUOTE]