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Old 05-24-2020, 08:29 AM   #1
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Average speeds

We are new at this and are traveling from Vegas to Milwaukee in a DP. I know this varies based on personal choice but I’d like to hear what the rule of thumb is for interstate average speed I can expect to travel safely. Also is there a sweat spot for RPMs to achieve the best efficiency and power for hill climbing? Thanks. John
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Old 05-24-2020, 08:52 AM   #2
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I plan for 60mph average on interstates. We usually roll at around 65. With fuel stops and quick on and off at rest areas we almost always nail it right a 60 avg. Obviously big city traffic throws a wrench in the numbers.
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Old 05-24-2020, 08:53 AM   #3
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Old 05-24-2020, 08:56 AM   #4
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We are new at this and are traveling from Vegas to Milwaukee in a DP. I know this varies based on personal choice but I’d like to hear what the rule of thumb is for average internet speed I can expect to travel safely. Also is there a sweat spot for RPMs to achieve the best efficiency and power for hill climbing? Thanks. John
Internet speed? Roughly 186,000 mph! ;-)

But you probably meant interstate highway speed. I'm usually near the speed limit on the road. Mileage varies as a rough function of the square of the speed, IIRC (always a dangerous assumption!).

I think that, pretty much by definition, the torque peak is the engine's most efficient RPM. For hill climbing that plays well to the diesel engine's low torque and power peaks, but for gas you wind up at much higher RPM to get enough power, which is not a particularly efficient place to be. You (well, I) likely do not care much about that in a gasser because you'd like to top that hill some time this week. In either case you will probably want to be at or near the power peak, though. For my Cummins ISL, that's 2000-2100 RPM. For the old Winnie it was something closer to 5000-5500 RPM.
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Old 05-24-2020, 09:06 AM   #5
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Our DP's sweet spot is about 63 mph.

And, in thousands of miles of travel while going cross country three times, we've found that the most we average per day is about 50-55 mph of travel time - that's because we stop to fuel, stop to pee, have to accelerate and decelerate when stopping/starting and it ALL detracts from your actual daily miles travelled per time period.

But, for efficiency and max. engine performance, we CRUISE at 63 ish.

Tail winds are a bonus and the blunt front end is a minus but, the scenery is always great and entertaining!
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Old 05-24-2020, 09:34 AM   #6
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I set the cruise at 65 on 4 lane roads and 60 on 2 lane

6 hours per day target travel time
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Old 05-24-2020, 09:35 AM   #7
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Caterpillar published some fuel efficiency and horsepower data awhile back.

From that: A 34K lb coach, at 60mph, needs to make 126hp to overcome aerodynamic and rolling resistance on a flat road with no wind.

At 75mph it needs to make 219hp.

From this we can conclude that fuel efficiency will decrease when traveling at 75mph as compared to 60mph.

Uphill RPM’s will differ from engine to engine. I manually select a gear, either 4th or 5th, that allows the engine (ISL) to maintain ~2000rpms. Don’t bother to look at fuel consumption at this time.
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Old 05-24-2020, 09:36 AM   #8
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I've always tried to research and find where the engine develops peak torque and run near that, or just a bit past that. Since we first started running interstates with RVs in the late '70s I've tried to stay as close to 70 mph as possible when circumstances would allow it. That placed the Cummins and Perkins diesel engines around 1900-2000 rpm and the 454 powered Flair at about 2600 rpm. Selecting the correct gearing at purchase time is critical to the mph-rpm relationship.

I always tried to keep the diesels below 2500 and the 454 below 4000 and never had a problem with hills anywhere within the US including AK, BC, or UT. We have made a coast to coast trip every year except two since the late '70s. I figure our avg speed at around 50 mph which allows for fuel stops and a stop for lunch with a short walk, and sometimes a short nap.

Every one is different and will have different driving habits. I'm thinking you will find yours soon enough. The important thing is to stay safe. We've been blessed with that so far.

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Old 05-24-2020, 09:48 AM   #9
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Our sweet spot is about 63 mph and with stops during the day ( average 300-400 mi. per day ) we figure for a 50-55 mph average. and fuel mileage with 300hp Cat is 7-8 towing Jeep toad.
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Old 05-24-2020, 09:54 AM   #10
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Originally Posted by jrj89134 View Post
We are new at this and are traveling from Vegas to Milwaukee in a DP. I know this varies based on personal choice but I’d like to hear what the rule of thumb is for average internet speed I can expect to travel safely. Also is there a sweat spot for RPMs to achieve the best efficiency and power for hill climbing? Thanks. John
Guessing you meant Interstate Speed - these engines are all just a little different - but a good rule of thumb might be 1750 rpms - i usually do it by sound and seldom look at the RPM's anymore. Your average speed at the end of the Day will likely be about 55 - most drive between 62 - 72 MPH going sown the road.

What is your Route? 15 to 70 or 80? Different drives - different Mountains - so need to have a little more info.

JMHO,
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Old 05-24-2020, 10:22 AM   #11
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What Interstate speed you choose, is up to you. Most states you can probably run 70. With that said, I set my cruise to 63mph. On days where there is a lot of open Interstate, we average about 60 mph when doing math for arrival time. On Interstates with more traffic and hill climbs, we average about 55 mph.

For the simplest math averaging....I use 50 mph which usually gets us there a little head of schedule, but allows for any issues that might arise.
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Old 05-24-2020, 10:48 AM   #12
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The RPM sweet spot for fuel efficiency is usually the lowest Rpm that enables you to stay in the transmissions top gear (6th on most coaches). The transmission and engine will cooperate on that and find an engine Rpm somewhere between the max hp Rpm and Max Torque Rpm. You can find it experimentally by running up to cruising speed and then slowly backing off until the transmission is forced to downshift a gear. Once you identify that spot, maintain speed a couple mph above that and the engine will smile. On most Class A motorhomes, it's 60-64 mph. The Rpms will vary quite a bit from engine model to engine model. On my Cummins ISL it was 1600-1650 rpms.
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Old 05-24-2020, 10:55 AM   #13
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Just completed a1000 mile trip set cruise at 63-64 thru Wa and 1/2 of OR with very little traffic. I would kick the cruise off on hills where I knew it would down shift just did manually. It was around 2500 rpm and gave me a 7.8 mpg average for the whole trip, I thought not to bad for an old V10 with the 4 sod tyranny (with the Banks trans controller. I personally think 70 is just to fast for me.
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Old 05-24-2020, 11:14 AM   #14
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67 for a average of 62-63... only possible by long days with only fuel stops. Last summer we left CO with a 40 mph tail wind all the way across Kansas and then a 30 mph through MO... averaged 70 mph and still got 1 mpg better than normal.
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