An Aid to Understanding a Transmissions Tow/Haul Mode - RV-Dreams Community Forum

Terry and Jo · RV-Dreams Family Member

An Aid to Understanding a Transmissions Tow/Haul Mode

Luvglass · RV-Dreams Family Member

Just like it says in the owners manual Terry. :)

Terry and Jo · RV-Dreams Family Member

Yeah, Fred. I know. BUT, you have to keep in mind that the F450 is my first pickup with a diesel and the first transmission with the Tow/Haul. I might have had years of professional driving, but some things just tend to be new and you know how some guys are with owner's manuals and directions.

Terry

Bill and Linda · RV-Dreams Family Member

Good link, Terry

FWIW: There is one take away in the link that is not in the manual as pertains to the Allision / Duramax. The comment about the maximum mileage towing (or not towing for that matter) for the Allision / Duramax is spot on.

At 1600 RPMs, in 6^th gear, which is “second overdrive,” (5^th gear is an overdrive as well) you will get right at 59 GPS confirmed miles per hour and also the best diesel mileage possible for a given overall weight. All of this on flat ground naturally with no headwind.

1600 is the torque point “sweet spot” for the Duramax / Allision combination up through 2011. All the GM / Chevy Diesel trucks (2500 / 3500 that is) have the same rear end ratio. So all these trucks should perform the same (best as to miles per gallon on flat roads, etc.) as regards this 1600 RPM number. That’s what the measured graphs say anyway and it has turned out to be proven in my tests over almost 5 years. It was for my last 1,200 mile trip last week.

I don’t know what the “magic” RPM number is for the Fords as they have different transmissions, some with only 4 gears, as well as many different rear axle ratios. But for any given diesel truck and transmission, there is a “sweet spot” as to economy and it is usually the lowest running RPM point of the engine in the top gear. Might be fun to find yours if you’re into this kind of stuff.

Bill

53 Merc · RV-Dreams Family Member

One other consideration to remember. If you are out of Tow/Haul because you think being in OD is more fuel saving, you might be pumping more fuel out the exhaust because you are in the wrong gear for the terrain/load/conditions. It has been my experience that mileage "seems" to be marginally better if I leave it in Tow/Haul and set the cruise at about 58 to 60 mph. Yeah, sometimes it is not in OD, but sometimes it is. At all times, it is in the best gear.

'member dad telling you not to lug the engine?

I have found over the years the computer that is updating in fractions of a second way more information than I can process in a week is the better way to go.

Bill and Linda · RV-Dreams Family Member

Ken, you are so correct. Mine, in Tow / Haul, does run in 6th gear (2nd OD) most all the time on flat ground or short grades while towing the rig.

Tow / Haul really was a great addtion to automatic transmissions. Even when you manually shift the gears the computer won't allow you to over or under rev the engine. The ECM in the Allison, and some others, are just a new world from what we used to deal with. Almost impossible to lug an engine with the Allision - especially in Tow / Haul. It will just protect itself and the engine no matter what you do to it.

Bill

Terry and Jo · RV-Dreams Family Member

Bill and Ken,

You guys are really helping me here. Now I guess I need to spend more time on the diesel truck forums, specifically Ford related ones, to see if I can find more about the "sweet spots."

Keep the comments coming as I am learning quite a bit. Like I've said before, I've had experience with farm diesels and 18-wheelers, but this whole thing with a Tow/Haul is a new learning experience.

Thank you both very much.

Terry

Bill and Linda · RV-Dreams Family Member

Actually, Terry, your 18 wheeler experience is like mine. You knew to shift gears in a particular RPM range and to keep the engine RPM’s “up” above a minimum level so as not to lug the engine. You had a manual transmission and so it was up to you to not let the “revs” go below a certain “lug” point depending on the engine type and the gearing of the rear end among other things.

Tow / Haul basically does the same thing. It moves the shift points such that the engine won’t lug under heavy loads. But it also, sort of, “locks up” the torque converter, which is sorta like the spring clutch in the manual transmission. (I’m being really general here, not specific so other experts can bear with me, please.) Because of that “lock up” there is less “slippage” between the transmission’s “input” from the engine and its output to the drive shaft. By limiting the “slipping” of the torque converter, in addition to changing the shift points, the Tow / Haul mode reduces transmission temps, or keeps them from increasing more than they otherwise would, because there is less slippage and therefore less heat. No different than when they told you “don’t ride the clutch!” on the old 18 speed Fuller we use to jam.

This is a pretty simple way of explaining it as I am a pretty simple guy. But maybe this helps a bit as to your understanding.

BTW, the issue with the Ford’s (not knocking them, let’s not get that started) is that until 2011 Ford didn’t really have as robust a competitor to the Allison transmission. Therefore Ford put a lot of different rear end ratios in the trucks depending on the GVWR / CGVWR ratings. Because of that and a differnece in engines,I just don’t have a clue as to what a good RPM number is for the F-450 you’re driving. But it will usually be, if things were set correctly, the RPM you engine settles in after shifting into the highest gear you have when in Tow / Haul. That’s the sort of rule of thumb as I have learned.

Keep in mind that an increase of but 1 mile to the gallon while towing is like getting well over a 10% reduction, or more, in the price of fuel. Do the math. That's a good piece of change for just watching the RPM's IMHO.

Take all this for what is worth. No arguments – just a conversation.

Bill

Terry and Jo · RV-Dreams Family Member

Bill,

Since I am obviously new with the F450 and the Tow/Haul transmission capability, everything that I can learn, both pro and con, is of great value to me. Thus, I take no offense with any thing you have said. In fact, I really appreciate all that you more experienced guys can tell me about them.

My F450 is a 2008 King Ranch with the 4:88 rear end. I wanted that configuration for the capability of towing in the mountains. Back in the '70's when I was driving 18-wheelers, my favorite was a Shupe Brothers truck with a 335 Cummins, a two-stick 4 by 4 transmission, and 5:29 rear differentials. More than once I was accused of being empty because I was gaining speed going up hills with a load. She weren't fast, but she were steady!!

While those were somewhat good times, they weren't conducive to a really successful married life with a wife and two kids. I saw too many truck drivers with ex-wives around to want that, either for my family or for me.

I must have done something right as we've been married for more than 42 years and have two sons of whom I'm very proud.

Terry

Bill and Linda · RV-Dreams Family Member

Terry:

Yea, I quit driving for much the same reason but the driving, while very important to the job, was actually secondary to my actual responsibilities. Buyt you did had a handfull with that semi's gears.

BTW, the GM / Chevy's have a 3.76 rear end so you can see what I was saying about rear end ratios, etc. being so different between different brands. Things are different as to exact operating "numbers" as pertains to RPM's etc., but the concept is still valid.

You will recall, back when they lowered the speed limits to "save gas" back in the 70's, all the complaints from truckers because the trucks were not "geared" for those slower speeds (55) and actually used a lot more fuel going slower - just like today if you don't run in top gear, all else being equal. Naturally, that is a generic statement.

Safe shifting

Bill