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Old 01-15-2022, 06:43 AM   #1
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Fuel efficiency vs speed - What is optimal speed to drive - It depends ....

I wanted to pin down for myself what speed to drive
It seemed like a simple question ,,, what is the most fuel efficient speed
It turns out I'm seeking a simple answer to a complex problem
I have a class A DP 2008 Tiffin Phaeton Cummins Freightliner 360HP (CM2150 8.3L)
The research made my brain bleed ... it depends ....
My conclusion ....
- How you accelerate is more important than speed ... just ease around
- 63 mph seems to be the anecdotal consensus although I'm not sure there is definitive research to support that particular number ... as again .... it depends

- has anyone definitively found the bona fide holy grail of current research that accurately answers this question or is the answer ... the impossible dream ... I suspect the definitive answer is ... it depends ...
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Old 01-15-2022, 06:58 AM   #2
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You nailed it!

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Old 01-15-2022, 07:02 AM   #3
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The most efficient speed for my rig would be different than for yours, so there would be no way to tell you. Experiment a bit with your rig to see if you can find a speed you like. At the end of the day though, it won’t matter. It will change depending on circumstances such as wind, hills, AC use, rolling resistance, temperature. You’re chasing a ghost. You’ll find that trying to be frugal by using an efficient speed is a waste of your time. Keep your tires properly inflated and your rig maintained. That will save you more fuel than that one perfect speed.
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Old 01-15-2022, 07:03 AM   #4
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The only fact on potential mileage is that using brakes costs mpg as you are converting motion to heat.
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Old 01-15-2022, 07:20 AM   #5
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My 1 year experience with a diesel rig tells me there is a spot for minimum speed where more downshifts begin to occur. I suppose this might be because the ideal torque rpm is not met when traveling too slow. I also know that when traveling above 65 the rig begins to really lap up the fuel. So about 62mph seems about right for my WinView.
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Old 01-15-2022, 07:30 AM   #6
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Caterpillar published a study, Understanding Coach/RV Performance. Here’s one page. It shows HP demand for various weight coaches at various speeds. One conclusion they reached was that, all else being equal, an increase in speed from 60mph to 70mph, increased fuel consumption by .8mpg.
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Old 01-15-2022, 07:35 AM   #7
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We are not driving a Class A but thought I'd throw my 2 cents in. Our 36' Ford F550 based C with a 6.7 Powerstroke seems to like 62-63mph. After that, we start incrementally losing MPG.
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Old 01-15-2022, 07:47 AM   #8
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In lieu of a long wished for adjustable gain control for cruise controls that would allow for slowing on upgrades without so much downshifting, I often drive the rolling hills with the cruise control disengaged and allow the vehicle to slow on many grades without requiring a downshift. That saves a lot of fuel.
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Old 01-15-2022, 07:47 AM   #9
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Quote:
Originally Posted by swduns View Post
- How you accelerate is more important than speed ... just ease around
I disagree with this for vehicles used primarily on an open highway like most motorhomes. While it is true that full throttle acceleration uses more fuel than half throttle acceleration the fuel used is a small part of the overall fuel consumption.

Let's assume that your MH accelerates from 0-65 MPH in 30 seconds at full throttle and you cruise at 65 MPH. Let's assume that it takes 1 gallon of fuel during the 30 seconds and it took 1/2 mile to get to 65 MPH. Then you drive 3 hours at 65 MPH for a total of 195 miles at 8 MPG. You used 195/8 = 24.4 gallons while you cruised and 1 gallon to accelerate. You then used 25.4 gallons to cover 195.5 miles therefore overall mileage is 195.5/25.4 = 7.7 MPG.

Using the same numbers assume that you accelerate a lot slower and use 1/2 gallon to get to 65 MPH and it takes a mile. You still used the same 24.4 gallons to cruise but 1/2 gallon to accelerate for a total fuel consumption of 25 gallons to cover 196 miles for a MPG of 196/25 = 7.8 MPG.

The difference is insignificant! A 5 MPH head or tailwind would make a bigger difference. Driving 62 MPH or 67 MPH would make a bigger difference.

For reasons of safety and to merge into traffic more quickly you might want to consider a higher acceleration rate to get up to speed on the highway. However your statement is true if you are in stop and go traffic. The lighter your throttle foot is the less fuel you use because virtually all the fuel you use is for acceleration and then you convert that fuel to heat in your brakes when you stop.

I take it easy when traffic is heavy but get onto the freeway as quickly as I can.
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Old 01-15-2022, 08:06 AM   #10
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I thought about that, the research I read considers drivers dynamically driving .... passing, grades, etc. .... if hitting a steady state and staying there ... would seem to align with your argument ... but some drivers are more aggressive and aren't in a steady state ... I think easing around includes setting the cruise control and trying your best to stay there. I've driven both ways. I think I will try to ease up to ~ 65, set the cruise control and not fight the traffic. I don't know the definitive right answer but that sounds reasonable to me.
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Old 01-15-2022, 08:14 AM   #11
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You nailed it!

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I agree.

Every rig has its own sweet spot. However, slower is more efficient until you get to very low speeds.

My rig provides about the same gas mileage from 35 mph to 50 mph. Then it starts to drop faster and faster.

By the time my rig reaches 70 mph consumption has more or less doubled. A 30 mph head wind does about the same thing.

Of course, applying brakes turns kinetic energy to heat, a total loss. That is why 55 mph highway driving is so good and 35 mph city driving is so bad.
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Old 01-15-2022, 08:25 AM   #12
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I agree.

Every rig has its own sweet spot.

Of course, applying brakes turns kinetic energy to heat, a total loss. That is why 55 mph highway driving is so good and 35 mph city driving is so bad.

On a side note……NOT using your brakes, and having a collision is more of a total loss than kinetic energy to heat.

Just my $0.02.
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Old 01-15-2022, 08:34 AM   #13
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In lieu of a long wished for adjustable gain control for cruise controls that would allow for slowing on upgrades without so much downshifting, I often drive the rolling hills with the cruise control disengaged and allow the vehicle to slow on many grades without requiring a downshift. That saves a lot of fuel.
X2. This is my approach.
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Old 01-15-2022, 08:51 AM   #14
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X2. This is my approach.

X3. Also use Economy Mode on the Allison to prevent many downshifts in rolling hills.
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