Slower for fuel economy?

[Dixieray53]

I'm curiuos...What device gives you instant fuel mileage..I have a trip computer, that gives me average mpg, instant mpg, and mileage to empty. How accurate these are I don't know...I do know I usually drive 450 miles and put in 60 gallons of fuel...7.5 everytime...Up hill, down hill, wind at my back, wind in my face, 55 or 70...MPG's are like the tree that falls in the forest...I would still like to know about the device..Peace...D

[Pairajays]

Determining the MPG of any vehicle is not an exact science except on a test tract. Too many variables, especially in an RV traveling across country or up and down one coast. I can't imagine accurate results with short trips at 55mph to 58mph, 60mph to 63mph or any other speed interval. Same thing between each each fill up. My diesel tank can not be filled to completely. There is a 10 to 12 gallon "bubble" that can never be used. When I get near capacity, I get 'blow-back' and takes forever before fuel fills the filler neck. I don't think I have reached that point and therefore I never "fill up". Because of that, my actual usage could vary by two to or three gallons. The mpg I get between 'fill ups" is irrelevant, it takes a lot of "fill ups" to get the true mpg. The mpg of my '02 36' HR Endeavor stabilized at 7.4 mpg after several thousands miles and never varied. 50,000 miles later, it was still at 7.4 mpg.

Jim E

[Pusherman]

I have about 3500 miles on the coach this year to date. Average MPG=8.4. Total mileage on the coach 23,000.
Just returned from a 350 mile trip and averaged 9.0, no toad.

Cat C7, driving about 62 mpg. I can usually get into 6th gear at about 56 mph.

Driving slower for me definitely improves mileage. When I do the back roads at 55+ mileage is really sweet. Highway I like to keep up with traffic, so I average about 62+ mph.

Coach does great at 70 mph, but the mileage drops to about 7.5+mpg.

[Triker56]

Originally Posted by Pairajays

Determining the MPG of any vehicle is not an exact science except on a test tract. Too many variables, especially in an RV traveling across country or up and down one coast. I can't imagine accurate results with short trips at 55mph to 58mph, 60mph to 63mph or any other speed interval.

If you are talking about my post with the different speeds and MPG.
They were gotten from over 9 years of keeping track of every gal. put in my MH on a spreadsheet.

And the average came from not just a fill up here and there. But over many during summer trips running near the same speeds the whole summer.

Total fill ups over the 9 years 122, with 9 year average of 10.232 MPG
This summer(10th) so far 1,295 miles average 10.398 MPG.
From 124.548 gal. bought for average price of $3.935 a gal.
Cost per mile 0.378¢

So none of my MPG were a guess from one tank or what a dash computer says. I do have VMSpc that is close on MPG but not as good as the spreadsheet.

I have noted since "08"-"09" the change to ULSD fuel in "10" has cost me around a .3-.4 drop in MPG at 55-58 MPG.
That is what my spreadsheet shows anyway. And I'm sticking to that.

Your experience may be different and there is nothing wrong with that.

[CD]

Originally Posted by HD4Mark

I understand that with a DP you can greatly improve fuel eco by going 55 instead of 65. Just for fun using a this Time, Speed and Distance Calculator so I don't have to do the math I plugged in the approximate distance we travel each winter to escape the snow. 1600 miles at 55 takes 29 hours 5 min. At 65 24 hours 37 min. So round trip losing about 4 1/2 hours each way we lose an entire day, is it worth it? We try not to be in a hurry but when going from cold to warm it it is nice to get there. On the way back, who cares.

I understand it is not possible to maintain a constant 55 or 65. I also will not go slower anywhere that I might aggravate other drivers. I don't like being behind someone going to slow so I won't put others through it. It also can make people make bad and dangerous decisions on passing.

So has anyone tried this and had enough fuel economy improvement to take the extra time?

It is much harder to average 65 than 55. The only way to do the math is to drive the trip at the 2 speeds and keep track of the time spent, then see what the actual average is. My guess is at 55 you may average 45/50 and at 65 you may only get 55. Your time gain at 65 may not be near as much as you think.

[Mr_D]

Originally Posted by Dixieray53

I'm curiuos...What device gives you instant fuel mileage..I have a trip computer, that gives me average mpg, instant mpg, and mileage to empty. How accurate these are I don't know...I do know I usually drive 450 miles and put in 60 gallons of fuel...7.5 everytime...Up hill, down hill, wind at my back, wind in my face, 55 or 70...MPG's are like the tree that falls in the forest...I would still like to know about the device..Peace...D

Most of the VMS line by Silver Leaf Electronics will do it. The VMSpc is software that runs on a desktop/laptop/netbook. It also requires a JIB (wire connection). The software is free, the JIB is $395. I've had one for many years, now run it on a netbook Velcroed on the dash.

[HD4Mark]

Got thinking about it and I'm not sure I could stand 55 on a 65 or 70 road. Ever notice when you have to slow down from 65 to a 55 zone it feels like you are dragging an anchor?

Damn the torpedoes full speed ahead. Or is it damage the Visa full speed ahead? Guess we are going anyway.

[Pairajays]

Triker56,

I did not not mean to question your results, just trying to make a point. You can't get accurate results with one fill after driving 55mph then another at 60mph.

Jim E

[Always-RVing]

You will lose approx 1 mpg for every 5 mph faster since (of course) it takes more energy to move the mass at 65 mph than it does 55 mph. Wind resistance accounts for 80% of the additional fuel used and about that much for all of the fuel consumed. Motohomes are a brick rolling down the street and a rolling earthquake.

Slower mph, tire psi, steady speed, tune-ups and lighter load are the best things you can do for improved mpg.

10 mpg @ 55 mph
9 mpg @ 60 mph
8 mpg @ 65 mph
7 mpg @ 70 mph
6 mpg @ 75 mph

Diesel #2[4]0.88 US gallons 129,500 BTU/gal kWh/Unit 37.95

Diesel engines burn almost 100%, but there's still only so much btu/k watts you can get from a gallon. Gas engines burn only about 65% of the gallon. The combustion chamber of a diesel engine gets around 1,000 degrees. That's why it will burn just about anything.

From Wiki

GAS Engines
Modern gasoline engines have an average efficiency of about 18% to 20% when used to power a car. In other words, of the total heat energy of gasoline, about 80% is ejected as heat from the exhaust, as mechanical sound energy, or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 20% of the fuel energy moves the vehicle.

DIESEL Engines
Engines using the Diesel cycle are usually more efficient, although the Diesel cycle itself is less efficient at equal compression ratios. Since diesel engines use much higher compression ratios (the heat of compression is used to ignite the slow-burning diesel fuel), that higher ratio more than compensates for the lower intrinsic cycle efficiency, and allows the diesel engine to be more efficient. The most efficient type, direct injection Diesels, are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 rpm.

[bucks2]

Interesting to note that peoples mileage hasn't changed when diesel changed from 500 ppm of sulphur to less than 15 ppm in 2006. Also not noticed is the difference between summer and winter diesel in those areas that use it.

I notice Batman doesn't take into account the change in engine rpm and gear ratio in his table. The fact that many coach's don't get to the top gear at 55 isn't accounted for and therefore the entire table is just a guesstimate. There is a reason that manufacturers put a top gear, generally an overdrive gear into the transmission. Using the logic that slower saves fuel, and using the same logic that the table uses, then:

11 mpg @ 50
12 mpg @ 45
13 mpg @ 40
14 mpg @ 35
........

I could get 16 mpg if I drove 25 all the time. Why isnt' there a movement to double an Rv's mpg by driving even slower? What is magic about 55?

I'm sticking to the theory that we all have different combinations of coaches, drivetrains, fuels, etc. and that one way of driving won't fit all of us. I still get better fuel mileage at 63 than I do at 55 regardless of what any table, logic or science says.

Ken

[Coma]

Originally Posted by Pairajays

Holy-e -Moses, 9.3 mpg at 70 mph? Are you sure you did your math correctly? I can't get that kind of milage at 55 mph.

Jim E

Math is good. Really, it is a single data point on one trip and I think you make the point that a lot more data is needed. I like the spread sheet idea. It makes me hopeful that long term average will be closer to 9 than to 6.

In general, although there is much difference between drive trains the most difference is in driver technique. The driver is the single biggest factor in fuel mileage or lack thereof. Speed is one factor, of course, but other things come into play. I use the boost gauge to tell how much I am fueling. Unless higher needed I use about 15" to accelerate. I maximize cruise control use when ever possible and I have used econo mode. I would be interested in hearing others driving tips.

[GPW]

Years ago we had a device on our Van camper... I believe it was called a Compu Cruise.... It gave instantaneous mpg readings , and a total mpg from programmable point to point ... Very handy in determining the most efficient speeds... Maybe they still make something like this ??? It did lots of other things too ...

[BpK9Miami]

This is an interesting and on-going topic. In my case I have a DP (puller, not a pusher!) so I don't have many couches to compare to in an accurate way.

My experience over the past 18 months has been a low of 6.8MPG last year (towing a 28' trailer with a 3 series BMW) from Texas up and down the east coast and back, to a recent 400 mile trip (minus the toad) of 7.24 MPG. My journal reflects various speeds over multiple terrain and distance but all the ranges stay within the 6.8 to 7.24 range; frankly, it's frustrating.

I've tried to be careful and have tried the various little tricks; altering speed, ensuring the OD is engaged, ect. All to no effect. Nothing seems to get me a majic increase in MPG. A friend has suggested having Cummin's do a diagnostic on the MH but they want $400 just to plug in the laptop. Ouch.

Reading through these posts I'm floored that some of you are routinely in the 9-10 MPG range! If I could get that out of 1/4 of my trips I'd be dancing for joy.

[CD]

Originally Posted by Coma

Math is good. Really, it is a single data point on one trip and I think you make the point that a lot more data is needed. I like the spread sheet idea. It makes me hopeful that long term average will be closer to 9 than to 6.

In general, although there is much difference between drive trains the most difference is in driver technique. The driver is the single biggest factor in fuel mileage or lack thereof. Speed is one factor, of course, but other things come into play. I use the boost gauge to tell how much I am fueling. Unless higher needed I use about 15" to accelerate. I maximize cruise control use when ever possible and I have used econo mode. I would be interested in hearing others driving tips.

COMA
I think you have the right idea. A heavy foot (fuel and brake) is expensive. I have been working on that concept for about 5yrs. Here's some of my experience. 1T Dodge PU. It has 85K with over 50% brake pads. Got 10/12mpg before, now about 13 towing or hauling Slide In. Maybe 15mpg empty now 16/18. 91 Pontiac I got 20/22mpg before now I get 23/25 local and last summer I made 2 trips to Eastern Wa. (400mi round trip) I got 30.5mpg on 1 trip &31 on the other. 03 Honda Civic the best I got before was 35mpg and 30 local before, now I regulary get 40/45 on trips and 35 local (DW gets 29/30). Last weeekend we went to Eastern Wa ( about 700 mi round trip ,about 100+ local) and averaged 39mpg.