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Old 06-02-2009, 11:23 PM   #1
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Fuel mileage

If this is not the correct forum for my questions, would someone please re-direct it.

I am driving for the first time in the Rockies and really need some advice. When climbing long, steep grades with a diesel push 38-ft. RV with a toad, how do I get the best fuel mileage.

1. Should the cruise control be on or off?

2. Should I turn the cruise control off when down-gearing?

3. Should the retarder be on all the time in the mountains?

4. Should the retarder be on or off when using cruise control?

5. When climbing a steep grade, do I down-shift and increase the accelerator?

I've read all the manuals, and they don't answer these questions. I would really appreciate any help.
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Old 06-03-2009, 08:41 AM   #2
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Having just returned from Yellowstone I believe the answer to #4 may answer most of the other questions.
The cruise is automatically turned OFF by selecting the engine brake.
Not knowing your engine and transmission types, the other questions would be hard to answer.
For the most part we did not use cruise in the mtns. How would you be able to "get up a head of steam" on the downhill in hopes of making it up the next hill?
Also the transmission electronics will probably do a better job of down/up shifting than most drivers.
H. Miller
BTW - Doubt seriously that fuel mileage will be your primary concern in mtn. driving
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Old 06-03-2009, 08:55 AM   #3
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1. When driving in mountains or even very hilly country you should have the cruise off.

2. you should have your cruise off,, but I think if you manually down shift it will shut the cruise off.

3. It should only be used to slow the vehicle down.

4. See post above.

5. Your transmission will automatically down shift when it requires more torque. Let the transmission do the work for you.

A good rule of thumb is whatever gear you use to go up a mountain is the gear you should be in when going down.

Good luck and enjoy the scenery
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Old 06-03-2009, 08:58 AM   #4
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In most vehicles, if you use the brake in cruise control, cruise will be turned off. In most diesels, if cruise is on, the retarder is turned off, but a touch of the brake pedal will change that and turn cruise off and allow the retarder to function again. So, you can leave the retarder switch on and use cruise as appropriate. However, cruise does nothing for you in mountain driving, i.e. you aren't "cruising", so it may as well be off.

In general you should let the transmission do the shifting and just push on the accelerator to maintain the speed you want. You only need to manually downshift if you notice the engine temperature rising more than several degrees. Downshifting will increase engine RPMS, which usually improves cooling.
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Old 06-03-2009, 11:36 AM   #5
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Cruise Control Off, if the transmission starts to hunt and won't stay in one gear, downshift one to bring the engine torque up.
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Old 06-03-2009, 04:26 PM   #6
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short pulls and long pulls

My motorhome has a Cat C7 engine with an Allison 6 speed transmission ... I have found the following to be true

a) Short pulls (let's say 1/2 mile or less) it doesn't make a whole lot of difference

b) Long pulls (more than 1/2 mile) then it is important to manually downshift to keep the RPM's above 2250 for power and to keep the temperature in the normal range. Naturally the steeper the grade the further you need to downshift ... and it helps if you downshift early in the climb

On long pulls if I do not keep the RPMs above 2250 the temperature rises beyond the normal range

Different engines will have slightly different thresholds ... if you watch your tachometer and your temperature gauge you will be able to figure out the thresholds for your engine and allow you to climb hills easily with your rig

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Old 06-04-2009, 11:44 AM   #7
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I also keep an eye on the transmission gauge. It may heat up faster than anything else.
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Old 06-04-2009, 01:07 PM   #8
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Different Priorities in Mountains

I failed to mention in my earlier posts that fuel efficiency is about the last thing that you worry about in mountain driving.

Your highest priority is getting down the grades safely. That generally means that you need to keep your speed under control without relying on your brakes too much ... when you do apply them ... apply them firmly in short bursts ... do not ride your brakes.

The next highest priority is getting up the grades without overheating your engine (or your transmission)

The next highest priortity is keeping your passengers and yourself comfortable ... by keeping your speed under control so that it is easy to negotiate curves ...

And way down at the bottom of the list is fuel economy. You may only get 3 or 4 mpg in some stretches of mountain driving ... and expect to average about half your normal driving speed in the more difficult stretches so leave yourself plenty of time ...

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Old 06-04-2009, 10:11 PM   #9
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Smile Fuel Mileage

Quote:
Originally Posted by geaugeausmom View Post
If this is not the correct forum for my questions, would someone please re-direct it.

I am driving for the first time in the Rockies and really need some advice. When climbing long, steep grades with a diesel push 38-ft. RV with a toad, how do I get the best fuel mileage.

1. Should the cruise control be on or off?

2. Should I turn the cruise control off when down-gearing?

3. Should the retarder be on all the time in the mountains?

4. Should the retarder be on or off when using cruise control?

5. When climbing a steep grade, do I down-shift and increase the accelerator?

I've read all the manuals, and they don't answer these questions. I would really appreciate any help.

1. Off

2. Yes. Your equipment may do this automatically, or not. Some cruise controls will actually do the down shifting, but I would prefer to be in control since this downshift may occur at too high a speed. Note that many (most?) engine brakes do not work while the cc is engaged and that the cc is turned off if the brakes are used (at least in all I've been in).

3. No, but if in doubt, use it. Not all mountain driving is up and down.

4. That's up to you. I use both cc and engine braking as a tool when they help me, not an all or nothing thing. Most modern transmissions are quite good at selecting an appropriate gear and I've never had my engine temperature get out of the normal range, even pulling up a mountain at full throttle. Now, that said, who'll bet it'll now act up!

5. See #4 above. If needed, sure. Otherwise let the computer do it's thing. I've heard cat owners say they need to downshift and keep rpm's up to avoid overheating. That's never been an issue with my Cummins

Drive carefully.

JT Kirby
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Old 06-04-2009, 10:56 PM   #10
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Guess it all depends on your engine/trans configuration.

I run a /Spartan K2/ISM/Allison 4000/2 speed engine brake/ and spend 6 months/year driving the Colorado mountains up to 12% grades with a toad in tow all over the state and particularly the Summit County area. At 1mpg on these uphills it's not fuel economy but safety/control that matters the most.

On DRY firm pavement, my rig does hills best (most control, safest) if I leave the cruise ON and engine brake on HI whether I am going up or down regardless of the grade.

NOTE: the following writeup applies only on DRY, firm pavement. Wet pavement and/or gravelly/sandy/dusty pavement and using engine brakes can be a recipe for disaster. In these conditions you will HAVE to shift more braking to the service brakes and less to none on the engine brake.

In my case, the cruise/ECM takes care of everything (engine brake, shifting, cooling fan brake booster) far more smoothly than my foot can do. It never exceeds 2400rpm on engine brake or lugs lower than 1300 w/o a downshift. Trans temp and engine temp barley move even if the grade is 5-10 miles up and another 5-10 down and even if the ambient is 95++F.

Some details:

Going up say a 6-7% the rig on full cruise naturally slows to roughly 40mph (and downshifts automatically around 1300 rpms or so as needed) which at the crest is about the right speed for the entire same grade decent which the engine HI brake can hold un-assisted by the service brakes. So I let it do all the work.

Going up less than 6% I'll back off the cruise to about 45-50mph on the way up (or I'll be too fast at the crest), keep the engine brake on HI and still let the cruise/ECM do all the work both up and then down, uninterrupted by me. My ECM will "throttle" the engine brake even if set on HI as necessary to maintain the cruise speed. My foot does nothing up and down.

Greater than 7% or so I still let the cruise do it all on the uphill, where I'll top out at 30-35mph,. On the downhill the engine brake can't hold these grades by itself so I'll have to occasionally tap (never ever ride) the service brakes to supplement the HI engine brake (thus disconnecting the cruise) and then I'll manually finish out the rest of the decent on HI, of course no cruise, and occasional service taps.

On my rig, without a doubt, the cruise/ECM is far superior in vehicle control on the decent, up to the engine brake's limit of about 7%. Steeper decent than that I'll have to help it with service brakes and then do the decent manually.

Of course, using cruise or manual, the cresting speed is the most critical. You want to crest AT (or less than) the speed you want to maintain all the way down. A DP isn't like a car. Needless to say, a 2 lane road gets a far, far, far slower crest speed than a 4 lane divided.

So, how you do it all depends on your rig. But, regardless of rig, the cresting speed is paramount - that you do not exceed the downhill speed desired at hill crest.
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Old 06-05-2009, 11:56 AM   #11
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Thank you for the many helpful answers. As a female loner I frequently run into questions that my be ridiculous to more knowledgeable folks, so it is good to have this forum.

I'm going to let my ignorance show once again. Where is and what does the transmission guage look like? The dash looks like it belongs in an airplane, and I thought I knew what all the guages are, but I don't find one that says "transmission." Would that be the temperature guage? If not, what?

I'm not really worried about gas mileage, but think if there might be a way to help it, I would like to know. I am very careful, very,very careful with my speed. The air brake retards down to second gear when going downhill. I do keep a close watch--if this thing got away from me, hoooboy!

Again, thank you so very much.

Anne
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Old 06-05-2009, 02:48 PM   #12
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It would really, really help to know the:
1. year
2. make & model
3. engine
4. transmission
of your vehicle.
A transmission guage reads the temperature of the trans. fluid and should be near the engine water temp guage. Our Monaco is calibrated from 140 to 320 degrees and runs in the 170-180 neighborhood and is labeled "trans. temp".
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Old 06-05-2009, 03:15 PM   #13
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Anne,
You may or may not have an actual transmission temperature gauge on the dash - many coaches do not. An in some, there are ways to display additional information on another display, such as the back-up camera display. If you have something called Triptek or Road Relay, you can probably call up things like the transmission temperature on a window on that display. If you don't have any such thing, don't worry about it and just use your engine temperature gauge on the dash.
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