Explain This

[RTOYZ]

I have a weekend warrior fs3000, On a normal trip we have 4 quads, 20 gals of water in the tanks, lots of food and gear and fill the truck bed with fire wood. On average to the coast I get 12-12.5mpg. Last weekend I went down to So. Cal and picked up a small 16ft trailer that couldn't have weighed more than 5k, I got 11.9 pulling that thing home with nothing but a small suite case in the bed of my truck. HOW IS THIS POSSIBLE???

[DuneJunkie]

What speed were you running?  Same speed as on a dune trip?  I have heard that fuel (yes diesel) and where you buy it, some makers of diesel burn differently, which cause better or worse mileage.............

[Towman 1]

Well I can tell you what is wrong, but you might not agree.

Living here on the central Oregon coast, when you go to Washington, you are going "UP" to Washington. When you go to California, you go "DOWN" to California. Now the way I am thinking, if you drove down to southern California and hooked on to that trailer, then you had to come up to get back to Oregon. Driving uphill takes more fuel.

Does this make sence ? 

[RTOYZ]

I was running the same speed as always, and yes it makes a little sense that coming back from Cali, you are going uphill, but to get worse milage pulling 9k less seems crazy. My mechanic thinks the fuel down there is worse and that could be a part of it.

[DuneJunkie]

My mechanic thinks the fuel down there is worse and that could be a part of it.

I have done lots of reading on Diesel fuels since I bought mine.  There differant manufactures do make differant "grades" of diesel, some burn hotter, some burn cooler, resulting in better or worse fuel economy. 

I know that I read a guy had done a test where he filled his truck with diesel from one company and made a drive.
The next trip he made the same direction, same truck/weight, and his egt's (exhaust gas tempertaures) were up, his mileage was down, and a few other things, the only differance was the type of fuel he put in the night before his trip.

I want to do some more reading on diesel fuel, and try to find out, what company is making better diesel.  I am interested to know if Fred Meyer diesel (i don't know where they buy it from) (which is 3 cents cheaper in town) is any better or worse than Cheveron, Texaco, 76, or Shell.

Anybody got any info?

[DuneJunkie]

Here is some information I found doing some research....................................

POWER
 Power is determined by engine design. Diesel engines are rated at the brake horsepower developed at the smoke limit.1 For a given engine, varying fuel properties within the ASTM D 975 specification range (see Figure 5-2) does not alter power significantly. For example, in one study seven fuels with varying distillation profiles and aromatics contents were tested in three engines. In each engine, power at peak torque and at rated speed (at full load) for the seven fuels was relatively constant. However, if fuel viscosity is outside of the D 975 specification range, combustion may be poor, resulting in loss of power and fuel economy. 

FUEL ECONOMY
 Here again, engine design is more important than fuel properties. However, for a given engine used for a particular duty, fuel economy is related to the heating value of the fuel. Since diesel fuel is sold by volume, fuel economy is customarily expressed as output per unit volume e.g., miles per gallon. Therefore, the relevant units for heating value are heat per gallon (Btu per gallon). Heating value per gallon is directly proportional to density when other fuel properties are unchanged.
ASTM specifications limit how much the heating value of a particular fuel can be increased. Increasing density involves changing the fuel's chemistry – by changing aromatics content – or changing its distillation profile by raising the initial boiling point, the end point, or both. Increasing aromatics is limited by the cetane number requirement (aromatics have lower cetane numbers); changing the distillation profile is limited by the 90% distillation temperature requirement.

Combustion catalysts may be the most vigorously promoted diesel fuel aftermarket additive (see Chapter 7). However, the Southwest Research Institute, under the auspices of the U.S. Transportation Research Board, ran back-to-back tests of fuels with and without a variety of combustion catalysts. These tests showed that a catalyst usually made "almost no change in either fuel economy or exhaust soot levels."2 While some combustion catalysts can reduce emissions, it is not surprising that they don't have a measurable impact on fuel economy. To be effective in improving fuel economy, a catalyst must cause the engine to burn fuel more completely. But there is not much room for improvement. With unadditized3 fuel, diesel engine combustion efficiency is typically greater than 98%. Ongoing design improvements to reduce emissions are likely to make diesel engines even more efficient.
 

[chocks]

I was running the same speed as always, and yes it makes a little sense that coming back from Cali, you are going uphill, but to get worse milage pulling 9k less seems crazy. My mechanic thinks the fuel down there is worse and that could be a part of it.

Your mechanic (and dunejunkie) are right. Diesel quality varies greatly from station to station, even day to tday at the same station. I have filled my truck at Fred meyer before and got less that 10 MPG empty, the tank before I filled at the same Freddys and got over 18MPG.
There is a truck stop on my way to work, that is where I always try to get my fuel..... usually fresh, and runs good in the truck

BTW: In Oregon there is no ASTM standard for Diesel or Avgas, therefore we are the dumping ground for "less than perfect" diesel in the northwest.

[superrulem]

well well well i think that the swomp gas in the night air was thicker on your way back
and that is the problem ha ha

[Towman 1]

Well after reading DJ's reply, I like mine better. lol  But I do think that buying diesel at different stations make a really big difference.

[chocks]

I really liked your explanation Tow.

[01350banshee]

Depending on your vehicle. If it's a Dodge diesel check your fuel filter. Happens to many of them including mine on occasion.