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Speed and MPG


Roadtrek 1

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How fast do you normally drive on the highway?   I've found that 60-65 is the best compromise even though I know 55 MPH would probably be best for efficiency?  

I'm certainly not in a hurry with the RV, but, I noticed if we were traveling less than 65 MPH large trucks would consistently pass us.   Sometimes..the big rigt trucks would be going faster than 70....or more...

What's your experience with this on the open road? 

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Speed is not the only criteria for fuel mileage. Speed is a major component. That written engine efficiency, engine RPM are important factors.  Each engine has a specific efficiency curve that indicates RPM and fuel usage. Modern diesel engines typically operate with least fuel usage in the 1300-1500 RPM range. Most of the big trucks are geared to run 65-68MPH. If run less than that speed fuel mileage goes down. 

Fuel mileage will need some investigation specific to your vehicle.  Good luck.

Bill

Bill & Lynn Baxter

MCI102A3 Conversion, Detroit Diesel S50  

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I drive no more than 65 MPH or observe the speed limits if they are less than that (such as the state-wide 55 MPH in California while towing). On the interstate highways, if I drive less than 65 MPH it can cause traffic problems with other drivers doing dangerous things to get around us - especially in the east. We just finished pulling from Georgia to New Mexico via I-20/I-10. In most of west Texas the speed limit is 75 (80 on some stretches). I still stick to 65 for economy and safety reasons. Being retired with no deadlines, what's the hurry?

Rob

2012 F350 CC LB DRW 6.7
2020 Solitude 310GK-R, MORryde IS, disc brakes, solar, DP windows
Full-time since 8/2015

 

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Aerodynamic drag is a function of the square of velocity.  Therefore, relatively modest increases in speed can have larger than expected impact on mpg.  All other things being equal (and they're not) the drag at 70 mph would be ~36% more than the drag at 60 mph.  

Sandie & Joel

2000 40' Beaver Patriot Thunder Princeton--425 HP/1550 ft-lbs CAT C-12
2014 Honda CR-V AWD EX-L with ReadyBrute tow bar/brake system
WiFiRanger Ambassador
Follow our adventures on Facebook at Weiss Travels

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37 minutes ago, Second Chance said:

I drive no more than 65 MPH or observe the speed limits if they are less than that (such as the state-wide 55 MPH in California while towing). On the interstate highways, if I drive less than 65 MPH it can cause traffic problems with other drivers doing dangerous things to get around us - especially in the east. We just finished pulling from Georgia to New Mexico via I-20/I-10. In most of west Texas the speed limit is 75 (80 on some stretches). I still stick to 65 for economy and safety reasons. Being retired with no deadlines, what's the hurry?

Rob

Rob, I agree with you 💯 percent.  I'm not in a hurry with the RV.... sometimes we've only gone 250 miles or less per day.   And, like you normally drive 60-65 MPH.  I'm retired as well.   

As for RPMs....the gearing on my Sprinter is over 2,000 when I'm driving.. I can't cruise at 1,500 RPMs  at highway speeds.  Maybe that's for big rig diesel engines? 

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Most often we travel at speeds between 55 and 60 . We generally travel US highways and avoid interstates . We wouldn't want to get in the way of anyone wanting to hurry to their end . ;)

Mileage ends up being 9 - 10 MPG . Our Monaco Monarch has a 315 HP V10 . 

 

Goes around , comes around .

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43 minutes ago, Roadtrek 1 said:

Rob, I agree with you 💯 percent.  I'm not in a hurry with the RV.... sometimes we've only gone 250 miles or less per day.   And, like you normally drive 60-65 MPH.  I'm retired as well.   

As for RPMs....the gearing on my Sprinter is over 2,000 when I'm driving.. I can't cruise at 1,500 RPMs  at highway speeds.  Maybe that's for big rig diesel engines? 

At 65 MPH pulling 14,000 lbs., our DRW F350 PowerStroke with a six-speed tranny and 3.73 rear end turns about 1,750 RPM. Coast-to-coast and gulf-to-northern border fuel economy averages 10.5 MPG. 

Rob

2012 F350 CC LB DRW 6.7
2020 Solitude 310GK-R, MORryde IS, disc brakes, solar, DP windows
Full-time since 8/2015

 

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20 minutes ago, Second Chance said:

At 65 MPH pulling 14,000 lbs., our DRW F350 PowerStroke with a six-speed tranny and 3.73 rear end turns about 1,750 RPM. Coast-to-coast and gulf-to-northern border fuel economy averages 10.5 MPG. 

I understand.... your engine is at least twice the size of my 3.0 V6.... with a 4.10 axle ratio. I certainly understand why your RPMs are lower.  

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2 hours ago, docj said:

Aerodynamic drag is a function of the square of velocity.  Therefore, relatively modest increases in speed can have larger than expected impact on mpg.  All other things being equal (and they're not) the drag at 70 mph would be ~36% more than the drag at 60 mph.  

36 percent more... that's pretty significant...

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I drive a 2007 Ford F-350 6.0L SRW, I get between 15 - 19 mpg at 1500 RPM's, at 1500 RPM's I am doing about 59 - 61 MPH not towing, with towing I get between 12 - 15 mpg.  However, when I switch to Tow Haul mode my RPM's shift to 2000 and I am still doing 59 - 61 MPH, I then drop to 11 - 14 mpg.

David

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We drive whatever the posted speed limit is, adjusted down for road conditions.  We only drive on interstates when absolutely necessary, so our driving is on the U.S. and state routes to get us from place to place.  Speeding tickets are not part of our budget so we follow the law.

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2 hours ago, Roadtrek 1 said:

Rob, I agree with you 💯 percent.  I'm not in a hurry with the RV.... sometimes we've only gone 250 miles or less per day.   And, like you normally drive 60-65 MPH.  I'm retired as well.   

As for RPMs....the gearing on my Sprinter is over 2,000 when I'm driving.. I can't cruise at 1,500 RPMs  at highway speeds.  Maybe that's for big rig diesel engines? 

An engine is the most efficient at or slightly above it's peak torque speed.  This is down around 1400-1600 RPM for many of the larger diesel engines.  Smaller and gas powered engines develop their peak torque at higher RPMs.

I couldn't find the peak torque RPM for your engine, but it's likely much higher than this. 

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1 hour ago, docj said:

Aerodynamic drag is a function of the square of velocity.  Therefore, relatively modest increases in speed can have larger than expected impact on mpg.  All other things being equal (and they're not) the drag at 70 mph would be ~36% more than the drag at 60 mph.  

^^^^^ that^^^^ You beat me to it. Rolling resistance is linear, aerodynamic resistance is logarithmic. At about 45-50 mph and up,  the aerodynamic drag quickly becomes the predominant factor in fuel consumption.

 

 
 
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34 minutes ago, Lou Schneider said:

An engine is the most efficient at or slightly above it's peak torque speed.  This is down around 1400-1600 RPM for many of the larger diesel engines.  Smaller and gas powered engines develop their peak torque at higher RPMs.

I couldn't find the peak torque RPM for your engine, but it's likely much higher than this. 

The charts for the 2012 PowerStroke stock show peak torque (666.29 ft. lbs.) at 2,000 RPM. At 1,750, the torque is down to about 630 - 640 ft. lbs. In 2013 - and almost every year after - Ford changed the tuning and the torque ratings increased significantly. 

Rob

2012 F350 CC LB DRW 6.7
2020 Solitude 310GK-R, MORryde IS, disc brakes, solar, DP windows
Full-time since 8/2015

 

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I am not retired yet so time is more of an essence with us than mileage. Having said that, the sweet spot on efficiency vs economy on our engine (per Cummins) is 1400 rpm. I ordered the truck with gearing to run 45, 55, 65 and 75 mph in the top 4 gears (10th, 11th, 12th and 13th) at that 1400 rpm. Fully loaded and with the smart car on board, we weigh about 48,000 pounds and average right at 7.6 to 8.4 mpg while running between 70 and 75 mph on the interstate. Lower speeds will increase the mpg somewhat but not that much. At 65 mph, I will average about 1/2 mpg more.

I could have increased this by a bunch if we had gone with a Cummins Advantage engine but I preferred the Performance engine instead to have the power when I needed it. At 500 HP and 1650lb-ft of torque, I can go up or down hills at pretty much any speed I prefer and with a sleeper cab, the ride is much more comfortable that the 6.7 F350 dually we started with.

2017 Kenworth T680
2015 DRV 38RSSA Elite Suites
2016 Smart Prime

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I think we're all comparing apples to oranges.  Let's see ...

Georgia Hybrid's 550 HP Cummins diesel pusher's peak torque is at 1400 RPM.

My 1999 Ford Powerstroke 7.3"s peak torque is at 1600 RPM

Second Chance's 2012 Ford 6.7 Powerstroke's peak torque is at 2000 RPM

Anyone else see a pattern here?  Where do you think peak torque is on Roadtrek's 3.0L Sprinter?  Above or below 2000 RPM?  😉

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54 minutes ago, GeorgiaHybrid said:

Lou, I don't have a diesel pusher, most of them remind me of a glitzy resort and not our style. We have a fifth wheel pulled by a Kenworth tractor that has a smart car on the rear deck of the truck.

So you have a rig that is as expensive, if not more, than a lot of us have with DPs.   We don’t do glitzy resorts.  So is it because we own a motorhome?  Do you feel the same about gas motorhomes?  

Barb & Dave O'Keeffe
2002 Alpine 36 MDDS (Figment II), 2018 Ford C-Max HYBRID
Blog: http://www.barbanddave.net
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5 hours ago, Second Chance said:

The charts for the 2012 PowerStroke stock show peak torque (666.29 ft. lbs.) at 2,000 RPM. At 1,750, the torque is down to about 630 - 640 ft. lbs. In 2013 - and almost every year after - Ford changed the tuning and the torque ratings increased significantly. 

Rob

My 2019 Powerstroke is rated 935 ft. lbs. at 1800 RPM.  Big jump in just 7 years!

Mark & Teri

2021 Grand Designs Imagine 2500RL, 2019 Ford F-350

Mark & Teri's Travels

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20 minutes ago, Barbaraok said:

So you have a rig that is as expensive, if not more, than a lot of us have with DPs.   We don’t do glitzy resorts.  So is it because we own a motorhome?  Do you feel the same about gas motorhomes?  

No, I just don't like the style of almost every class A I have been in. The flashy ceilings, marble floors and fancy lights are not our style. I prefer the more "home" feel of a nice fifth wheel. We also have a class C that we use on shorter trips.

I don't give a RIP at what you drive. If you prefer a 1962 Rambler American, that is your choice, not mine so get your knickers untwisted and take your class A attitude back and stick it in a closet where it belongs.

Kirk and several others complain because the HDT people on this board don't post anywhere except on the HDT forum.  Your response is the reason we stay there and off the general forums.

2017 Kenworth T680
2015 DRV 38RSSA Elite Suites
2016 Smart Prime

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2 hours ago, Lou Schneider said:

Anyone else see a pattern here?  Where do you think peak torque is on Roadtrek's 3.0L Sprinter?  Above or below 2000 RPM?  😉

Lou,  here is link to everything you wanted to know about the OM642 engine, 

https://en.m.wikipedia.org/wiki/Mercedes-Benz_OM642_engine

The torque curve for this engine is 1,600 to 2,400 with peak horsepower at 3,800 RPMs.  It has 190 horsepower and 325 foot pounds of torque....

When I'm on the road it's usually running between 2,000 and 2,400 RPMs. 

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13 hours ago, GeorgiaHybrid said:

Lou, I don't have a diesel pusher, most of them remind me of a glitzy resort and not our style. We have a fifth wheel pulled by a Kenworth tractor that has a smart car on the rear deck of the truck.

Wow, so I'm pushing a "glitzy resort" down the road.  Here all along I thought I was hauling a Freightliner chassis with a Cummins diesel behind it, pulling a 24' enclosed trailer.  

I don't really have a "style".  We started in a couple different fivers but I couldn't have my Harley with me so we switched to this set up.  Come on by our camp site sometime and I'll have a beer with you.  Just don't drive that smart car over and park it next to my Harley please.

Joe & Cindy

Newmar 4369 Ventana

Pulling 24' enclosed (Mini Cooper, Harley, 2 Kayaks)

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