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 > Some Thoughts on Improving MPG..Driving Techniques

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JohnnyT

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Posted: 03/30/08 12:21pm Link  |  Print  |  Notify Moderator

I may regret this but I want to try and have an intelligent conversation about how to get the best fuel mileage. At least my thinking as to why I seem to be on the upper end of the mileage tree.

This thread is not a quick read nor are the links but I believe if your interested in improving fuel economy you might find the time well spent.

I am certain that what ever your fuel mileage is, there may be good chance to improve it. For this discussion purposes I want to ignore the speed issue since its a no brainier that if you lower you speed within reason your MPG should improve. What I want to talk about is driving habits. I will ask that those who participate on this thread please keep the focus on driving habits. Here is a companion thread Thoughts on MPG II.... Aero, Weight, Rolling Resistance

Anyone who has caravanned with me will attest that I am a lead foot... Meaning I will generally drive at the posted speed limits within reason. While I have driven at 75 my preferred speed in the 65 MPH range. I will maintain my speed conditions and traffic permitting whether I am going up or coming down a grade. I really think that the one variable that has not gotten a lot of air time is the length of runs. We are drivers not parkers so we will accumulate seven or eight thousand miles in a 4 or 5 week period.

The short version is all other issues aside if you want better mileage spend more time at a constant speed and the longer you do this the better your mileage.. The long version follows. [emoticon]

Please take note that my own fuel mileage assertions are based on read outs from the ECM via Trip Tek.

First a couple of preambles;

Wind and grade are contributing factors but not within our control.

The aerodynamics are also contributing factors also fall into they are what they are.

Tire pressure is a factor and within the control of the operator.

Rolling resistance is a factor but also falls into it is what it is.

Speed is a factor and also within the control of the operator.

Gear ratio's; Both the transmission and especially the rear ratio is a factor. But again they are what they are.

Aside from tire pressure and the speed we choose to drive the biggest factor we as drivers have the most control over is our driving technique and patterns.

Every engine, transmission, rear end ratio, weight rolling resistance of tires will have an optimum range in terms of RPM's where the best fuel mileage can be obtained. Here is a link from Cummins illustrating these issues. Cummins Power Spec in my case 1650 is a sweet spot based on my gearing and tire characteristics.

I think the issue is how can we as drivers most improve our mileage... Speed is certainly a factor but I think there is one that has not gotten too much air time... Driving patterns... My particular focus is the fuel it takes to get up to speed how often I have to do it...

The reason I bring this up is that I have had the opportunity to sit and discuss fuel mileage with folks driving identical coaches to mine as in same everything!!... Yet some get extraordinarily better mileage as indicated by the Trip Tek read outs. The range is considerable, from around 7.5 to 9.5 on average...Keep in mind that I am talking about identical coaches in terms of manufacturer, size, weight, HP, transmissions, rear end ratios, tire size, tire brand and dinghy weight...

Our motor home is a 40 foot DP moving about 35,000 pounds down the road including the dinghy. The engine is an ISL400 with 4:63 rear. The overall average for the 45,000 miles we have driven is 8.54 which includes a considerable amount of mountain driving. On trips or legs where there is little mountainous driving our average is around 9.5 we have also gotten extended periods where we have gotten over 10 illustrated via Trip Tec screen below... This is of course in flat country. There was little or no wind.

[image]

I believe the real reason for our mileage is due to our driving patterns... And maybe a little due to the aerodynamics of our coach. We tend to drive long legs with few stops... When stopping I look to minimize the distance I must drive to get fuel. I minimize idle time since the MPG is zero when idling. I always start trips with a full fuel tank I am careful to not allow the engine brake to bleed off too much speed where I will need to expend extra fuel to regain the speed where ever possible. Our average speed is generally around 65 MPH...

Every time I have to regain speed I can watch the instantaneous fuel consumption get in the 2 to 3 MPG... If I am starting from a dead stop it will take a considerable distance before the AVG MPG recovers...If the situation permits I will build speed gradually but usually that is not an option since I want to be up to speed relative to traffic as soon as possible. If there is no traffic I will be a little less lead footed coming up to speed.

We have a couple of trips where we have made repeated runs.. One is 270 miles and the other is 700 miles... The first 100 miles of these destinations is the same...In both cases we will average in the neighborhood of 7.7 for the first hundred and in the case of the 270 mile trip we will arrive at our destination with an average for the trip of 8.4 to 8.7 the difference is primarily due to the amount of traffic... The longer trip we will have the same 7.7 for the first hundred and will arrive at our destination with an average that ranges from 9.45 to 9.65... We have made the shorter trip 5 times. And the longer trip 10 plus times.

I think I am getting the optimum mileage given the speed I drive at because we tend to have long runs... And I focus on minimizing the number of times that I have to recover speed lost due to braking or due to stops.

There are other issues since I have observed differences that can only be explained by where I bought fuel and what type of fuel I was running. I get better mileage on summer blend then winter blend.

If you really want to understand what you can do to improve your MPG then spend some time understanding and considering the wisdom presented on these links. The Cummins in particular is aimed at over the roads trucks but there is plenty of wisdom to be had that applies to Motorhomes. The Caterpillar document is focused on Motorhomes...

Cummins MPG Insights

Caterpillar Understanding Coach/RV Performance

While I am keenly interested in a discussion I have little interest in the stop and smell the roses line of reasoning or the get out of my way or driving slower is a hazard rhetoric barrage... I am a card carrying member of the smell the roses fraternity. I retired in my early 50's so I could smell the roses.

As I already stated
We will generally make long runs,
We only stop for fuel around each 1150 miles...
Try to pick our stops where there is a minimum of stop and go,
Go easy on the brake not bleeding off too much speed that I will have to expend fuel to regain
In congested areas I try to go with the flow of traffic so I minimize speeding up and slowing down
I minimize idling time where the MPG is zero

While my data, comments, and references are focused on Diesel I believe the principles are the same for Gasoline powered units as well.

For those that are going to slow down or already have you will get the most benefit of improved mileage if you take stock of your driving habits and keep in mind the tremendous amount of fuel that is consumed accelerating.

Below is a representation from one of our 700 mile runs and beyond. Take note of how long it took for the mileage to creep up.

You will note that the entry for 804 to 904 appears twice this is a function of the way I uploaded the image... I had wanted to keep the log going for our entire trip but after a while doing so was more and more uninspiring to do. If I remember right our trip mileage for the slightly over 8,000 mile trip was 9.47 last years 7,800 mile trip was 9.54. No attempt was made to keep track of stops other than the first fuel stop. Just noted the MPG whenever someone thought of it after the first 700 miles.

Those of you will ECM readout capabilities and the patience keep a log at reasonable intervals doing it every 25 miles is not something I will ever do again.

In case you are wondering why the picture of the Trip Tek screen and the monotony of keeping track... I have been thinking about writing this post for few years just never got around to it but given the price of fuel and the interest on the forum I took the time to make this post.

The Trip Tek has three screens where MPG is tracked "Today", "Leg", and "Trip".

[image]
[image]

* This post was last edited 04/02/08 10:33am by JohnnyT *   View edit history

wolfe10

Texas

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Posted: 03/30/08 01:23pm Link  |  Print  |  Notify Moderator

Johnny,

Great FACTUAL introduction to "Improving MPG".

I will cover one additional "driver controlled" factor in some detail:

ALLISON MODE:

There are TWO TOTALLY DIFFERENT "LET THE TRANSMISSION CHOOSE THE CORRECT GEAR" MODES/PROGRAMS in the Allison ECU which is the "electronic brain" controlling shifting and other functions.

In ECONOMY MODE, the transmission will not downshift even at WOT (Wide Open Throttle) until the engine pulls down to peak torque RPM in some application and 200 RPM lower that "regular mode" in others. And similarly, on up-shift, economy mode will shift into a higher gear EARLIER-- with the difference only at or near WOT.

In REGULAR MODE, the transmission WILL downshift much earlier (to maintain higher engine RPM).

ONLY at higher throttle positions is there any difference, so on flat ground you will NOT notice any difference (except accelerating from a stop IF you are at or close to wide open throttle).

It can make a BIG difference in rolling hills. If you are in rolling hills and regular mode (particularly with the cruise control on), it is common for the transmission to shift down to 5th on the uphill and back to 6th on the downhill. Repeat this process hundreds of times. In economy mode, you will stay in 6th gear unless the hill is so steep or so long that the engine can not pull it without dropping below peak torque RPM. If you can pull a hill in a higher gear (lower engine RPM) AND the engine does not overheat, THAT IS WHAT CATERPILLAR AND CUMMINS RECOMMENDATION FOR THE MOST ECONOMICAL WAY TO CLIMB A HILL with a modern turbo, after-cooled diesel engine.

If you know you will need a lower gear because of the steepness of the grade and/or are engine temperature is rising higher than thermostatically controlled temperature, if driving in economy mode, use the down arrow to drop a gear (this is what I do) or switch out of economy mode. Be sure to switch back into economy mode when past the steep section, or agree to pump extra fuel in the tank.

IF your engine begins to overheat, your HP/weight ratio is low OR if it irritates you to loose a few mph on a hill in the name of saving fuel, in the hills, by all means drive in regular mode.

It confuses me to hear people advocate driving in economy mode only on flat ground, as there is not 1% difference in shift RPM's between regular and economy mode on flat ground, excepting accelerating from a stop if you use WOT.

Every time you start the coach, the transmission is in regular mode. This is the default setting. IF you push the mode button, it goes to "economy mode" AND the light illuminates.

There is no "absolute" on how much difference in fuel economy driving in economy mode will have. On flat ground where you will be in 6th gear irrespective of what mode you are in, there will be ZERO difference. The MOST difference in mileage will be in rolling hills, where in regular mode, particularly if on cruise control you will start up a hill in 6th gear, go to WOT in 6th gear, downshift of 5th gear still at WOT (WHERE IT IS USING A LOT MORE FUEL). After the hill is crested, the transmission will up-shift to 6th, then likely coast a little in 6th gear (unless you are driving with the exhaust brake on-- if you are it then applies the exhaust brake AND downshifts TOWARD the pre-select gear which is generally either 2nd or 4th).And so on 6-5-6-5-6-5-6-5.......

A modern turbo inter-cooled diesel is much more efficient at low RPM high throttle settings. This is NOT my opinion. It is stated in just those words in the Cat Owner's Manual and likely the Cummins manual as well.

Note: In either mode, you are free (and welcome) to use the up and down arrows to PRO-ACTIVELY choose the correct gear. You can not screw anything up-- even if you down-arrow to 1st gear at 70 mph, the transmission understands that you meant "please downshift to the next lower gear as soon as the engine RPM will not exceed the pre-set amount. Then downshift again when safe....."

By the same token, you can shift between regular and economy mode as often as you want with the transmission in any gear when you make the change.

OPINION: I drive in economy mode 99% of the time, including in REAL mountains, but use the up and down arrows to choose the proper gear. I use regular mode ONLY when I am willing to say, "I am willing to throw a lot of fuel away to gain a little performance." When passing on 2 lane roads, THIS IS the case.


Brett Wolfe
2003 Alpine 38'


FMCA Forum: www.community.fmca.com/index

Diesel RV Club:http://www.dieselrvclub.org/

tallyo

Fort Myers,Florida

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Posted: 03/30/08 01:49pm Link  |  Print  |  Notify Moderator

Sorry guys but the best, easiest way for most of us RV'ers to improve mileage is watch our weights and gas pedal. These RV's are great big wind collectors, resisters and are heavy.
Drive like you have a raw egg between your foot and the pedal. And I mean all the time, starting , going up hills, down hills, around corners, that is anytime you have to accelerate do it like an egg was there. Finally watch your speed.

I have talked with folks who have identical RV"S and their mileage is all over the charts. The cause is the weights and their driving habits.


Tallyo
2012 Phaeton 40 QBH
Freightliner, 6 spd Allison
2010 Chevy Equinox 2LT w/ Brake Buddy Vantage
Drinks-6, Eats-4, Sleeps-2
Semper Fi


hershey

Albuquerque,(fulltime) NM, USA

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Posted: 03/30/08 01:55pm Link  |  Print  |  Notify Moderator

Johnny thats the best article I've ever seen on this list. The links you eluded to were very informative also. I've cut and pasted this into my ongoing personal information file.
Thanks again.


hershey - albuquerque, nm
Someday Finally Got Here
My wife does all the driving - I just get to hold the steering wheel.
Face Book Group: All About RVing and We Fly RC's
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Briteskys

Mesa, AZ

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Posted: 03/30/08 02:04pm Link  |  Print  |  Notify Moderator

And yes, an excellent article and thanks to Brett for educating me about the 'economy' mode button.

* This post was edited 03/30/08 06:22pm by an administrator/moderator *


'02 Beaver Patriot Thunder
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JohnnyT

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Posted: 03/30/08 02:14pm Link  |  Print  |  Notify Moderator

Folks please limit the discussion to driving patterns, or Techniques to improve on fuel mileage...

JohnnyT Moderator

* This post was edited 03/30/08 02:22pm by JohnnyT *

KOG

Winterville GA

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Posted: 03/30/08 02:39pm Link  |  Print  |  Notify Moderator

No magic here. What you've said about driving at a constant speed, minimizing brake use, minimizing idle time, minimizing cold starts (long runs) is all valid. There are some sight differences in driving technique for gas vs. diesel engines, but the points you've already made remain valid. With diesels you can run at lower speeds without losing efficiency and you can run at near full output without losing much efficiency. Gas engines are very bad at idle, not good at low speeds and real fuel hogs when run wide open. They much prefer to be run at about 70% of full output at very near peak torque speed. Move off of the sweet spot with a gas engine and mileage drops quite a bit. Move off of it with a diesel and mileage drops a little. Also gas, buy fuel with as little ethanol in it as you can get.

Fuel consumption with generators is related. Diesel gensets are pretty efficient regardless of load. Gas ones like to be run at about 50-80% of full load. More or less load on a gas one and the efficiency drops off quite a bit.

wolfe10

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Posted: 03/30/08 02:44pm Link  |  Print  |  Notify Moderator

KOG wrote:

Fuel consumption with generators is related. Diesel gensets are pretty efficient regardless of load. Gas ones like to be run at about 50-80% of full load. More or less load on a gas one and the efficiency drops off quite a bit.


KOG,

Many good points. And while the statement above IS accurate, on the generator, one is not looking for efficiency, but for lowest consumption.

It WILL use more fuel to run two A/C's to bring generator up to 70% load than to run one.

chuck4788

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Posted: 03/30/08 02:58pm Link  |  Print  |  Notify Moderator

I use cruise control because it does a better job of keeping the speed constant, which saves fuel. But it does require looking ahead and planning to determine the best timing for a lane change or to disengage.

One problem I have with my cruise control is it is not very well behaved on grades: when first encountering a grade if the speed drops below 58mph it will downshift into 5th, but the shift is slow and I will often lose more than 5mph in road speed. Then the cruise control attempts to regain speed by accelerating up the hill. As a side note my tranny is the 4000 and Economy mode shifting reverts to normal shifting when cruise is engaged (light stay on but the shifting is like regular mode). On closed thottle it will always downshift to 5th at 58mph, but if I keep some thottle on I can run it down to 45 in sixth (well below torque peak) and it will accelerate. Me thinks Allison needs to improve their shifting algorithms.

My fix is to be pro active and disengage the cruise when approaching a climb and if a downshift is required do it with the keypad so there is very little change in road speed. I also let the speed sag while climbing and overrun on closed thottle on the down side (safety permitting).

I also do the other things previously mentioned such as:

minimize idle time.
minimize non hiway driving.
avoid stop n go driving situations.
accelerate as gently as possible,
minimize speed changes.


Chuck
02 Beaver
505hp C12 Cat
Trailblazer toad


JohnnyT

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Posted: 03/30/08 02:49pm Link  |  Print  |  Notify Moderator

wolfe10 wrote:

Johnny,

Great FACTUAL introduction to "Improving MPG".

I will cover one additional "driver controlled" factor in some detail:

ALLISON MODE:

There are TWO TOTALLY DIFFERENT "LET THE TRANSMISSION CHOOSE THE CORRECT GEAR" MODES/PROGRAMS in the Allison ECU which is the "electronic brain" controlling shifting and other functions.

In ECONOMY MODE, the transmission will not downshift even at WOT (Wide Open Throttle) until the engine pulls down to peak torque RPM in some application and 200 RPM lower that "regular mode" in others. And similarly, on up-shift, economy mode will shift into a higher gear EARLIER-- with the difference only at or near WOT.

In REGULAR MODE, the transmission WILL downshift much earlier (to maintain higher engine RPM).

ONLY at higher throttle positions is there any difference, so on flat ground you will NOT notice any difference (except accelerating from a stop IF you are at or close to wide open throttle).

It can make a BIG difference in rolling hills. If you are in rolling hills and regular mode (particularly with the cruise control on), it is common for the transmission to shift down to 5th on the uphill and back to 6th on the downhill. Repeat this process hundreds of times. In economy mode, you will stay in 6th gear unless the hill is so steep or so long that the engine can not pull it without dropping below peak torque RPM. If you can pull a hill in a higher gear (lower engine RPM) AND the engine does not overheat, THAT IS WHAT CATERPILLAR AND CUMMINS RECOMMENDATION FOR THE MOST ECONOMICAL WAY TO CLIMB A HILL with a modern turbo, after-cooled diesel engine.

If you know you will need a lower gear because of the steepness of the grade and/or are engine temperature is rising higher than thermostatically controlled temperature, if driving in economy mode, use the down arrow to drop a gear (this is what I do) or switch out of economy mode. Be sure to switch back into economy mode when past the steep section, or agree to pump extra fuel in the tank.

IF your engine begins to overheat, your HP/weight ratio is low OR if it irritates you to loose a few mph on a hill in the name of saving fuel, in the hills, by all means drive in regular mode.

It confuses me to hear people advocate driving in economy mode only on flat ground, as there is not 1% difference in shift RPM's between regular and economy mode on flat ground, excepting accelerating from a stop if you use WOT.

Every time you start the coach, the transmission is in regular mode. This is the default setting. IF you push the mode button, it goes to "economy mode" AND the light illuminates.

There is no "absolute" on how much difference in fuel economy driving in economy mode will have. On flat ground where you will be in 6th gear irrespective of what mode you are in, there will be ZERO difference. The MOST difference in mileage will be in rolling hills, where in regular mode, particularly if on cruise control you will start up a hill in 6th gear, go to WOT in 6th gear, downshift of 5th gear still at WOT (WHERE IT IS USING A LOT MORE FUEL). After the hill is crested, the transmission will up-shift to 6th, then likely coast a little in 6th gear (unless you are driving with the exhaust brake on-- if you are it then applies the exhaust brake AND downshifts TOWARD the pre-select gear which is generally either 2nd or 4th).And so on 6-5-6-5-6-5-6-5.......

A modern turbo inter-cooled diesel is much more efficient at low RPM high throttle settings. This is NOT my opinion. It is stated in just those words in the Cat Owner's Manual and likely the Cummins manual as well.

Note: In either mode, you are free (and welcome) to use the up and down arrows to PRO-ACTIVELY choose the correct gear. You can not screw anything up-- even if you down-arrow to 1st gear at 70 mph, the transmission understands that you meant "please downshift to the next lower gear as soon as the engine RPM will not exceed the pre-set amount. Then downshift again when safe....."

By the same token, you can shift between regular and economy mode as often as you want with the transmission in any gear when you make the change.

OPINION: I drive in economy mode 99% of the time, including in REAL mountains, but use the up and down arrows to choose the proper gear. I use regular mode ONLY when I am willing to say, "I am willing to throw a lot of fuel away to gain a little performance." When passing on 2 lane roads, THIS IS the case.


Brett, thanks for your contribution I was hoping for your contribution since you are our resident expert [emoticon] I do use the mode program and it does limit the number of times that the transmission down shifts into 5th. I believe doing so is a contributing factor in the MPG we experience.

The point of all this is that aside from lowering your speed your driving technique is a major contributor to the MPG you experience.

Just lowering you speed is not the whole solution if you want better fuel economy. It will surely help but if you want to optimize your fuel efficiency then you will need to take stock of your driving techniques to take advantage of the fuel efficiencies associated with driving slower.

As I mentioned I had the notion of this thread a couple of years ago and discussed the parameters with Brett but held off frankly because of the difficulties of having a focused discussion.

As I have already stated while the discussion and reference are specific to diesel powered motorhomes, I believe that the same principles apply to Gasoline powered coaches. Higher RPMS and acceleration devour fuel. So it behooves one if interested in fuel efficiency to adjust their driving techniques to keep the RPM's down and work to minimize the need to gain or regain speed.

JohnnyT

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