32 replies to this topic

[Rocky B]

When driving a DP when does one ( me ) start to just waste energy. I do know that it has been standard that any 5 MPH over 60 cost 10 to 15 cents added to the price of fuel. Would it be prudent to stay bet. 55 and 60 MPH when the posted speed limit is 70MPH. What steps could I do to save on gas (diesel) as much as possible. My 8 year old was upset when she noticed I was driving 65MPH. Does anyone else has a Monaco Diplomat with the cummins 330? I already use the cruise control as much as possible. What other tips and if you have an explaination would be greatly appreciated.

[Stanley P. Miller]

Find out what your most efficient speed is, most likely at your torque peak in 6th gear but might be a couple MPH slower. If you are willing to try it give a slower speed and fifth gear a try.

Put the transmission into economy mode unless you need power to pass or merge. If you let the transmission auto-shift on hills take it out of economy then too. The economy mode will also reduce the tendency to hunt between fifth and sixth at low RPMs and on cruise making life easier on the transmission.

Me, I's stuff the kid in the basement with a video game and cruise at my torque peak in 6th. :-)

[SIBERNUT]

They're just upset that you are enjoying what you worked for and won't be in your estate.

[Desolation Roe]

Me, I's stuff the kid in the basement with a video game and cruise at my torque peak in 6th. :-)

LOL! Now that's funny.

Geo

[dmcb]

They're just upset that you are enjoying what you worked for and won't be in your estate.

If an 8 year old is thinking that way, God help us all.

[Kirk]

. What steps could I do to save on gas (diesel) as much as possible. My 8 year old was upset when she noticed I was driving 65MPH.

Are you really serious, or just posing a question?

In most large RVs, once you find the most fuel efficient speed at which to travel, any increase of 5 mph will cut your mpg by 10% so the cost would be 10% more than you are currently paying, whatever that might be. This is the first time that I have ever heard of 10 - 15¢. I doubt that it would be 10¢/mile, but ..............................

[Technomadia]

Also remind your kid that you're moving your house... not a car

- Cherie

[AFChap]

Kirk is right ...the difference in cost would be % rather than cents. My dp would be below peak torque at 55, and would be in 5th gear most of the time, jumping back and forth between 5th and 6th. It does very well in the 63-66 range. And when I am in hilly country it actually does better if I bump the cruise up to 68 ...the slightly faster speed means it is less likely to have to downshift on the small grades, and when it does the quicker shifts=less lugging.

Tell the kid that is somewhat true for cars, but it also depends a great deal on how the vehicle is geared. I've had cars that are running flat out at 55, and others that were hardly going yet at 60. I really get irritated at all the global warming propaganda I see in many of the popular kids movies ...they are being indoctrinated, and too much of it is pure baloney.

[Rocky B]

It does appear that it is very happy chugging along bet. 60-65MPH. It's just that I have followed a few RV who remains at around 55MPH regardless that the posted speed is 70. Is it because they are driving a gasser? (How could you talk about indoctrination..... My kids are in a Catholic School)

[Bruce T]

I'm not so sure it's how fast you cruise but how fast you get to that speed. Gentle changes with your right foot will affect your consumption big time.

[Barbaraok]

Depending upon which state you are in, there may be a reason the RVs are doing 55, because it is mandated by law as in California - anyone pulling a trailer/vehicle is suppose to be at 55 mph. We always observe the limit for trucks, just makes sense to us. Otherwise, we cruise at about 62 - that seems to be her sweet spot.

Barb

[geysergazers]

Illinois also has a 55MPH speed limit for RVs and Trailers (trucks, too, but of course they mostly ignore it).

[Kellie Roberts]

The sweet spot on the Bird is about 62mph (DP) as well.

And as for the CA speed limits (and we have abided by their laws) -- from Mojave to where we are currently staying in Santa Rosa -- 55mph appears (from the truckers & such) to be just a suggestion.


PS Barb -- I'm printing your winery list out & we hope to add a few more to it while exploring everything we can while we're here for the week.

Depending upon which state you are in, there may be a reason the RVs are doing 55, because it is mandated by law as in California - anyone pulling a trailer/vehicle is suppose to be at 55 mph. We always observe the limit for trucks, just makes sense to us. Otherwise, we cruise at about 62 - that seems to be her sweet spot.

Barb

Edited by Kellie Roberts, 12 April 2012 - 10:55 AM.

[Stanley P. Miller]

It does appear that it is very happy chugging along bet. 60-65MPH. It's just that I have followed a few RV who remains at around 55MPH regardless that the posted speed is 70. Is it because they are driving a gasser?

A lot depends on the gearing, with a heavy trailer and light truck you need low gears to pull the thing, same sort of thing for motorhomes. That means your engine spins pretty fast at cruise speed and the difference in 55 and 65 can make a big noise and fuel use difference. That applies to both gas and diesel powered rigs.

Other folks just like driving at slower speeds, maybe they feel safer, like the scenery going by slower or want to save as much fuel as they can.

I'd swear I've met a few folks that were just trying to see just how many cars they can stack up behind their rig, might be part of an underground contest of grumpy old folks sharing Polaroid snaps of the lineups via snail-mail. :-)

[dmcb]

Personally I drive the speed I want. If its a70 speed limit, that is the max, not the minimum speed.
However when I am on a 2 lane and i see cars behind, I pull over at the first chance and let them pass.

[Ran D. St. Clair]

I wonder if this talk of a sweet spot at around 60 something has more to do with the feel and sound of the vehicle than anything to do with fuel efficiency? I am guessing that the most fuel efficient way to cover level ground with no wind is in the highest gear at a very low throttle setting, like about 25 MPH. Obviously that's not practical, safe, or even legal on most freeways. So the question is not about maximizing fuel efficiency, but finding the best trade off between fuel efficiency and all those other practical considerations. Depending on the road, anything under 55 is going to upset the other drivers around you. For me it comes down to getting where I want to go in a reasonable amount of time. I end up at about 62 MPH in the truck, but that has very little to do with fuel efficiency.

Stan, or others with a technical bent.... Can you confirm or deny that the most fuel efficient speed is actually quite low?

[Stanley P. Miller]

The speed that is most efficient is hard to figure and varies from vehicle to vehicle. Three main factors, engine efficiency at the required RPM, wind resistance and rolling resistance.

Wind resistance goes up quickly as your speed increases, https://en.wikipedia.../Drag_(physics)

Assuming a more-or-less constant drag coefficient, drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity.

Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW).

Rolling resistance changes are mostly from the additional torque needed to increase the speed: https://en.wikipedia...es_.28torque.29

For pneumatic tires, the direction of change in Crr (#Rolling resistance coefficient) depends on whether or not tire inflation is increased with increasing load. [43]. It's reported that if inflation pressure is increased with load according to an (undefined) "schedule", then a 20% increase in load decreases Crr by 3%. But if the inflation pressure is not changed, then a 20% increase in load results in a 4% increase in Crr.

Engine efficiency... What a can of worms, my cat has left more comprehensible messes under my recliner!



http://www.physicsforums.com/archive/index.php/t-165455.html

And some interesting observations and suggestions that should keep you busy for hours.

http://www.omninerd.com/articles/Improve_MPG_The_Factors_Affecting_Fuel_Efficiency

Buy the book http://www.amazon.co...e/dp/1560911999 or Google for a PDF copy of it.

Better yet buy the book and a good graphing calculator, hand both to the pesky kid and tell them they are grounded until they present hard numbers for all reasonable speeds. Kid might turn into an engineer and be able to support you in your old age. :-)

[Bob & Mary]

Time is money. If the incremental time savings is worth the incremental cost drive as fast as you want. Schools today are indoctrinating our youth with guilt for being successful. Try to counter with common sense (and good luck with that).

[Kirk]

It does appear that it is very happy chugging along bet. 60-65MPH. It's just that I have followed a few RV who remains at around 55MPH regardless that the posted speed is 70. Is it because they are driving a gasser?

The numbers that I quoted to you were for one of those humble gassers! We lived in one for almost 12 years and seem to have managed to be just as happy as those who spend more!

I'm not so sure it's how fast you cruise but how fast you get to that speed.

No doubt about it, you can make a pretty major difference in fuel consumption by your habits of acceleration and deceleration as well. But your cruising speed does also. It takes a lot of time and careful study to determine just what cruising speed is most efficient, but it is pretty safe to say that if you accelerate hard and then rush up to each stop light or sign, using lots of brake to get stopped, you will burn much more fuel than you save by traveling slowly.

Depending upon which state you are in, there may be a reason the RVs are doing 55, because it is mandated by law

This also is something to be considered as tickets can quickly run up the costs of travel!

I wonder if this talk of a sweet spot at around 60 something has more to do with the feel and sound of the vehicle than anything to do with fuel efficiency?

While it is true that I never measured my fuel efficiency for a complete tank of fuel at 25-30 mph, I did do so for speeds ranging from 55 mph to 70 mph for at least three tanks in succession, and using mostly interstate highway travel for most tests. I have also done that same thing for US routes at 55, 60, 65, then at 58 and 62. Over the past 14 years I probably did mpg tests at least 100 times with various average speeds and conditions. I feel pretty confident that my coach would not have been more fuel efficient if I had traveled at 25 or even 30 mph. To do so, I would never have traveled in overdrive at all and would have spent a great deal of time running in second gear. Simple math of engine revolutions per mile traveled should make it fairly apparent that the most efficient speed will be at some point fast enough to spend the majority of time in the highest gear available.

[richfaa]

We are in a TRuck and 5th wheel but it is the same. We drive at 62/65 towing because that is where we feel comfortable.If the speeed limit is below that then we are at that limit. Don't know about fuel enonomy good or bad at that speed and it is not a issue. We also do our best to stay within the legal speed limits solo as we do not like speeding tickets.