Best practice for steep downhill freeway grade - Fiberglass RV



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Old 05-26-2019, 02:08 PM   #1
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Best practice for steep downhill freeway grade

Although I've towed boats in the past, I'm new to towing a trailer. Next week I'll be practicing with my new tow vehicle and new to me Casita, and want to ask about a downhill freeway grade that will be part of most of my trips.

When I used to drive our old Class A RV down this grade, I would often shift out of "OD" into regular D, using the engine to brake the rig a little. My new Durango has regular drive, and then an option to shift into manual, which would then allow for a downshift. Which is better, to do that, or to use my brakes, thereby of course using my trailer brakes, even if I end up using them quite a bit while going down the grade?

I hope this question makes sense. I thought it would be good to ask for advice rather than start experimenting at 55 mph!
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Old 05-26-2019, 03:04 PM   #2
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I never use overdrive when towing, ever. My truck has a tow mode which automatically downshifts on down grades.

Read your owners manual and see what it says on using overdrive when towing.
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Old 05-26-2019, 03:15 PM   #3
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Thanks. I'm understanding it better now.
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Old 05-26-2019, 06:39 PM   #4
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Best practice for steep downhill freeway grade

You will definitely want engine braking, not friction braking only, on a long down grade. As said, read your manual regarding transmission use when towing. Most likely you’ll use the manual mode and downshift as needed.

Best practices vary a lot with modern transmissions because much depend on the control software. Mine says to turn off OD on hills, but okay on flat terrain.
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Old 05-26-2019, 06:46 PM   #5
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I drove up to the Grapevine and practiced a bit, so now I feel a lot more comfortable. Next week get to take my Casita out driving around to practice towing and backingm. First trip not til 6/21 due to other family things.
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Old 05-26-2019, 07:32 PM   #6
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Whoa, the Grapevine is certainly steep practice.

It's a good idea to practice. I like to go to a big parking lot (mall? closed supermarket?) If you have cones they are good markers for backing up. Go slow, don't be afraid to get out and look what's happening as often as you need.

You got this.
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Old 05-26-2019, 07:39 PM   #7
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One of the reasons I chose a Toyota Sienna over a Honda Odyssey was that the Toyota has an auto-transmission that allows you to downshift. I use it to apply engine braking on downgrades when towing. The trick is to slow down so as to to exceed the limits of your brakes and tranny. The brakes will always engage but can overheat and then you get "brake fade." The tranny, on the other hand, will not shift if the engine RPMs exceed safe limits when you downshift, so keep it slow enough to allow the tranny to shift to lower gear and save the brakes from overuse. I guess those figures are available, but with a little practice I can judge it pretty well. If you do find your speed downhill increasing to the point that you have to stay on the brake pedal risking brake failure, and the tug's computer will not let you downshift, then hit the brakes firmly and quickly slow to a speed where you can downshift and let up on the brakes. The added engine braking should allow your brakes to cool and recover. Use both tools (tranny and brakes) to control your downhill speed.
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Old 05-26-2019, 07:52 PM   #8
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Quote:
Originally Posted by thrifty bill View Post
I never use overdrive when towing, ever. My truck has a tow mode which automatically downshifts on down grades.

Read your owners manual and see what it says on using overdrive when towing.
My truck has 3 overdrive gears!
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Old 05-26-2019, 09:46 PM   #9
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Thanks all. Trailer practice is next weekend, need to find some cones. Will be having experienced person help me with initial set up of matching my TV and Casita up, and adjusting my brake controller
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Old 05-27-2019, 06:43 AM   #10
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When in cruise my transmission automatically downshifts if my speed reaches 4-5 mph over the setting to hold the cruise setting. 2015 Ram 1500. My 2012 Chevy did the same thing.
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Old 05-27-2019, 10:03 AM   #11
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Quote:
Originally Posted by thrifty bill View Post
I never use overdrive when towing, ever. My truck has a tow mode which automatically downshifts on down grades.

Read your owners manual and see what it says on using overdrive when towing.
That's what I've read and been told. In fact, the local RV mechanic has told me to NEVER use OD when towing. That said, it's very tempting since I am towing a 3000# trailer with a 2002 Chevy diesel pickup that seems to be loafing along even with the trailer behind it.

Any comments/thoughts about this will be appreciated. Thanks to all.
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Old 05-27-2019, 10:27 AM   #12
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When towing I am more concerned about pulling with a loafing engine than one running at high RPMs. With my tear drop I rarely used the tow mode, but the Casita is heavy enough that I use it all the time. Does effect milage some but not as much as I thought it would. Id rather end my trip with a sound tranny and engine than trying to squeeze a few extra miles per gallon out of my rig.
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Old 05-27-2019, 10:48 AM   #13
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So now I'm curious, since my vehicle doesn't have have a "tow driving" mode (does have a full tow package). What does tow mode do, exactly? Does it basically just automatically put the vehicle in a lower gear on downhill grades, or is there more to it?
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Old 05-27-2019, 11:06 AM   #14
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Downshifting

Most all tow vehicles can be downshifted. exception being a Highlnder Hybrid. but then it has dynamic braking, using the electric motor as a generator as it recharges the battery.
Rule of thumb is to use the same gear going down the hill as you need for going up.

If the grade is too long and/or steep. apply the brakes, but with intermittent harder braking, letting off so the drums or disks can cool. With hard braking they will not get hot as fast as with continuous light applications.
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