Fuel economy

[Petervs]

While driving along on my way to POG2, I started thinking about a way to impress my good friend Brian ( owner of an 8V92) with the fuel economy of my Series 60. We both have a ProDriver installed, and it shows the average fuel economy obtained since the last reset.

My curiosity was really, how high could the fuel economy go in a completely best case situation? It seems to me that this would be just driving along at a steady speed, no wind, no hills, always in the highest gear of the transmission. Well, of course a higher number would be obtained if going down hill with a tailwind, but that would not be realistic.

I live in Washington State, and it seems that no matter where we go, the road has hills. In regular driving, with traffic, various accelerations and decellerations, over the course of a tank of diesel, I usually average right around 7.5 MPG. That is my 45 foot Marathon, loaded with everything we use, and towing a VW Golf at 2500 pounds.

So, I filled up the tank in Winnemucca, NV, reset the Pro Driver, hopped on I-80 and set the cruise control on 55 mph, and let the traffic roll past ( the speed limit was 75!). I did this for 80 miles and watched the fuel economy average creep up until it got to 9.32 mph! There were a few hills, but not much wind. It was a real world situation. I am amazed that the economy is this high considering we have a house going over the road.

I took a picture of the display at 9.25 mph and emailed it to Brian with a link to one of those magnetic fuel conditioner websites, and told him how good it was working.....he did not bite.

Anyway, I think that gives a kind of upper limit on the efficiency of operating the coach. On the way home, I did it some more at different speeds to see how the faster you go burns more diesel.

95 miles at 62 mph in Oregon yielded 8.71 mpg, no headwind.

65 miles on I-84 with a 7 mph headwind in Northern Utah at 65 ,mph yielded 7.45 mpg. There was one big pass.

92 miles at 66 mph near Boise yielded 7.6 mpg, 5 mph headwind


136 miles at 71 mph on I-85 in southern Idaho yielded 7.13 mpg. 8 mph headwind.


Conclusion:

At $3.00 a gallon, you can minimize operating cost by driving 55 mph, and you will pay 33 cents a mile, or if you hustle at 71 mph, you will pay 42 cents a mile. A 1000 mile trip would cost $90 more at the higher speed.

My idea was just to put this all in perspective. Your thoughts and comments are always welcome.

[Jon Wehrenberg]

At the risk of waking up JDUB, Peter has spoken volumes.

As an owner you can choose to save time or money, but not both. Fast is clearly going to allow you to save valuable time, while going a little slower saves not only fuel, but is safer.

When Fast Roger and I headed west for the rally we cruised at about 70. I lost mileage because we were heading into the prevailing winds, I was towing the equivalent of a tank in the form of a Hummer, and we were going from a few feet above seal level to the higher western elevations.

Going home we set our cruise control on 62 and as Peter pointed out our mileage climbed. What hurt us on the return was we encountered severe winds, mostly on the nose that made the coach shudder, and were strong enough to flex the top of the entry door outward enough so Di could see it flex.

I will not try to preach or advocate anybody drive slow. That is a personal choice, but I am certain we would have hit two cars in the rear on the return if we were driving at 70. Both put us in conditions beyond our control. The first was a lady in a Focus that suddenly and without warning spiked her brakes when we were hit suddenly with gusty winds and a torrential downpour. I was lucky the adjacent lane opened up so I could avoid her. The second was a small pickup merging that failed to notice the end of the merge lane was occupied with a stalled truck, and he opted to jump immediately in front of us, instead of slowing until we passed. He was all of a sudden in front of us going very slow and we used heavy braking. At 70 I would have run over him.

[Jerry Winchester]

Nice analysis and perspective. Jon will take exception to my comments, but Roger won't And I had already typed this response before Jon posted his, so I didn't change it or wake up

So then the 1000 mile trip at 55 would be +/- 4 hours longer, which depending on what you have going on isn't much especially if you are driving in an area devoid of traffic.

Since Roger belongs to the "I don't know where I'm going, but I hate to be late" crowd, I have also reconciled myself to the "drive the speed limit" view and pay the extra bucks. I kinda view the bus like I do the airplane; I didn't buy it to save money on fuel, I bought it to go fast. Or in the case of the bus, as fast as the appropriate limits allow.

Lastly, there was a large debate in Texas over the raising of the speed limit in West Texas to 80 mph. In the end, the safety issue wasn’t the speed, it was the differential in speed meaning if everyone was driving along about the same speed, there were less issues (insert accident here) than when someone was driving 70 and someone driving 55. Now I know this was not the point of your post, but it was going to get around to this eventually, so I just added it for the sake of bothering the Luby's crowd.

Blast away.

And welcome Pete. I enjoyed meeting you at POG II and look forward to your posts. I think this is a good forum for your technical skills.

[Jon Wehrenberg]

Jerry, Despite the Geritol bottle dents in Fast Roger's coach front end I have to make a few points.

I was in the lead. I often had to slow down so Roger could catch up. He couldn't have gotting the prune pit and Geritol bottle dents if I was following.

I agree with the speed differential issue, and anyone who drives through Ohio knows exactly how scary it is. Our two near misses on the return were not related to a posted differential or related to our choice to drive home slower. Both instances were small vehicle drivers who were completely unaware of the fact that the speed differential they created put us in a very difficult situation, and had our speed been greater the outcome may not have been as it was.

We have driven hard and fast, but now at this point being gainfully unemployed we adopt the attitude that we are already "on vacation" when we get in the bus, and the destination is just a point along the way. Hurrying down the road just robs us of time in the coach.

[Chris and Debbie Yates]

Here is a comparison of fuel consumption with and without a trailer.
JW and I have similar coaches, both 40' with 8v92s.
We both fueled up in OKC on the way to POG11, after 543 miles we fueled again in ALB. Jerry needed 91 gallons, mine took 115. I was drafting Mango Andretti all the way so I guess it could have been worse. This got me to thinking - if running in a convoy, if you are not leading causes you to use more fuel because you are constantly modulating the throttle How did you California boys do in your six coach train?

AP

[Jerry Winchester]

I also ran my generator from about 11:30pm to 8am. When I downloaded my DDEC info, it says my coach gets 6.5 mpg but driving with AP and MM, it got 6, so that must be the difference.

We were also running 71 mph more or less.

[Petervs]

There is a place in the Pro Driver Set up where you can enter a correction factor to get the fuel economy to come out right. The DDEC brain has lots of sensors, like engine rpm, road speed, fuel flow from tank to engine and from engine back to tank. The computer has to be told things like the axle ratio and the tire diameter, and if everything is exactly right, then you get the correct read out on the Pro Driver.

You have to have the Pro-Link or Nexiq Download reader Jerry showed us in order to reprogram all the variables, but the Pro Driver has one place where you can enter a correction factor to force the result to come out right.

Remember, the Pro Driver was designed for truck drivers working for a large fleet, and they did not want to give the driver much control, only the fleet manager is supposed to have access to some factors. But they figured the Pro Driver would not be accepted by drivers at all if they could not make it read right. Hence, the correction factor in the Setup page.

If you compare your reading to the actual mileage on several fillups, then you can enter a correction factor, and after 1-3 tries you will have an accurate number. No reason to have incorrect data displayed all the time.

In my case, my correction factor is 1.00, and the result happens to correllate within 0.1 mpg or so every time, unless I use the generator a huge amount which rarely happens.

Detroit Diesel has a web page for the Pro Driver, also people on the phone who are very helpful. If you don't have a user manual for it you can get one from the website.

http://www.detroitdiesel.com/product...nics/index.asp

[merle&louise]

Peter:

Thanks for the information from your experiment. I have always wanted to do a test like that, but never took the time. The results are about what I thought they would be.

I used to drive 70 - 75 most of the time if the roads were good (in Louisiana we have to drive 20 - 25 mph). A few years ago, I had a blowout on the driver's side front tire at 65 mph on I-25. Luckily, no damage occured; I was able to pull over and stop. 5 or 10 mph can make a big difference in reaction time. Now I drive 65 mph and I feel much safer.

On the way home, we passed thru El Paso to visit a friend. On I-10 (6 lanes) a woman passed me doing about 70. I was driving about 55 because we had just pulled onto the interstate. When she was about 500 yards in front of me her driver's side rear tire blew out! I hit my brakes, she fish taled right, then left, then spun around and did a 360! She managed to stay in the left lane and not hit the concrete barrier. When she stopped and got out of the car she was visiably shaken up. If I had been driving 70 we would have both sustained some damage.

Jon is right; slow down, be safe, and enjoy these beautiful machines that we are fortunate enough to own.

[Jon Wehrenberg]

I nominate Peter for the Pro Driver seminar in April.

This is the kind of stuff we need to learn.

[ken&ellen]

Although that is a Harley term referring to center of gravity and slower cruising speeds I apply it to our Liberty. I do not have Pro Driver or any type of computer style fuel monitor, but I do have a pencil as well as a calculator. My 8V92 averages 6 mpg and drops to 5 - 5.5 towing. I like Jon set the cruise control on 62 mph for fuel economy as well as for safety. We are on the road most weekends as well as all vacations racking up the highway miles. I would rather get to our destination in one piece vs set a land speed record. Ken & Ellen