If you don't mind my taking this on a tangent - why do I get 7 to 8 mpg when the rest of the fleet gets 6.3? Same 15 liter Cummins, same gear ratios. Mountains or flat, heavy or light, I rarely drop below 7 and never stay there.

 

It has a lot to do with personal driving habits. I started driving different the last month or so and for those weeks so far I have been averaging about 1.25 to 1.5 mpg better than the fleet average. You could give some drivers the most well equipped well speced truck on earth and they would get worse than you or the rest of the fleet just because of the way they drive...but they would always blame it on the truck or company speed policys or something.

 

I'm with WIND on this one! I've said it all before. My company cut me back from 80 to 75 some time ago. The SAME hill climbing west out of Knoxville, TN (actually, the one up to the Cookeville plateau) I USED to do in 10th gear with a 79k lb load, at about 1500 rpm and ground covered in say 3 mins at close to 70mph.

NOW, I'm in 9th gear, (sometimes lower) at about 60mph, 1800 rpm, and maybe 1 or 2 mins longer to reach the top!

ANYWAY you slice it, that's MORE revolutions of the engine (fuel BURNED with each) for a LONGER period of time! And, I DO all those mountainous western states, and rollercoaster southwestern states every week! I KNOW I spend MORE time in 9th gear than I USED to.

Considering that the max speed limit out there is 75, and I RARELY exceeded that when the truck would do 80.... the ONLY difference is that I now must downshift SOONER. Also..... the truck USED to be able to "cruise" at 60 mph in a 55 zone in 10th gear at lower rpm's.... but now MUST be shifted down to 9th gear at near limiter rpm's or it will lug out and die! :lol:

I've READ the physics on resistance OVER 65 mph, and I MIGHT have to believe it. But, that is a different story. IF you are gonna run higher speeds than that, the fuel economy will STILL be relative to the GEAR you have to run in.

I'm no scientist, engineer or mechanic by ANY means.... but I KNOW that the whole purpose of rear end gear ratios is to provide either PULLING power or cruising power. The MORE time spent in a higher gear at lower rpm's the less energy is expended, less fuel burned, and less wear on the engine.

As for Malakai saying he runs LESS rpm's at 64 than at 68, I can SEE it if in the same gear. But, if you spend MORE time in a lower gear in the mountains than you USED to.... well..... there goes the fuel economy!

 

Golf and Wind:

You are both 100% correct about the moment you're on the side of the hill. You're using the same fuel going slower.

However, that is only part of the picture. We need the average.

The average would include the flat part before going up the hill, the ramp up before hitting the hill, the actual hill itself, then the crest.

What I'm saying is going slower lowers the average horsepower required for that whole operation.

Going faster might seem to improve mpg for one of the 5 factors, but you're neglecting all of the energy it takes to actually hit the hill faster.

I've done mid 7's across I-80 in pa.

 

Now, think about it. The original poster lives in WV. How many flat areas do you find there? it's all hills. You're spending 8 minutes going up a hill, then only 2 minutes going down. The fuel used per minute going up is not made up for when you go down because you do not spend enough time going down to make it up.

Like I said. In FL, we were doing better on fuel economy. But once we get out of FL, we're running I-77 and I-81 a lot. The additional fuel we burned in the mountains was not offset by the savings in flat areas. Over all, our fuel economy went down, not up. If your trucks are running across OH, IN, IL, IA, and NE, you will see better fuel economy. But, point the nose up and you've got Niagra Falls supplying your engine.

 

I dont agree with the slowing of the trucks for MPG. I find it to be corporate jargon to help their insurance rates. After speaking to numerous mechanics and drivers it has been proven that cutting back MPH does not help MPG. In all honesty it seems that trucks are using more fuel!!!! HMMMM then why is diesel going for $5.05 per gallon up here in good ole NY and everywhere around the country, if trucks are using less fuel?

I dont know. But one thing for sure its more corporate and political crap and peoples agendas.

 

Allan5oh said:

No.... I'm using MORE fuel per minute due to higher RPM's.... AND I do it for several minutes LONGER.

MAYBE.... if on a flat road JUST before the hill. But, the ones I'm talking about are preceded by a downhill "coast" where the LIMITER keeps me from going over 75.... actually SLOWING me down on my approach, and DELAYING the moment where I can start the acceleration up the hill. By the time I can effectively accelerate, the weight has killed my momentum and I have to downshift almost immedately (where I USED to could actually coast further up the hill and then MAINTAIN the higher speed AND higher gear. (And be up and over the hill in LESS time with lower rpm's.

There was no fuel "lost" going 80 on flat land, cuz I would rarely go over 70 or 75 anyway.... and still CAN. And there was no fuel lost coasting at 80 or more DOWN the approach hill. What is LOST, is the KINETIC energy I was storing on approach. :x

You're assuming "expended" energy from flat land to hit the hill faster. I'm talking about "free" kinetic energy that I no longer have.

OR, I'm talking about a long slight grade BEFORE the hill, which I used to could pull in 10th at LOWER rpm's that I NOW have to pull in 9th, at HIGHER rpm's for a longer period of time.

And I'VE done 6 over I-40 from coast to coast. NOW, on the same route, I do about 5! Some of those long slight grades out West eat up ALOT of time in 9th, where I used to could run in 10th!

FURTHERMORE..... the 5mph drop has added approximately 3 full hours to the trip each way. 6 more hours total at (or over) 1500 rpm. Even at an "average" of 65 mph, and 5 mpg, that's 13 gallons per hour X 6 more hours = 78 more gallons at nearly $5/gallon = $390 MORE per round trip.

$390 X 20 team trucks per week = $7800/wk X 52 weeks = $405,600 MORE in fuel costs per year for my small company. And that's JUST for the "EXTRA" time the truck is running at highway speeds. If you take the 1 mpg loss that I BELIEVE we are experiencing, each average trip being 5500 miles, takes 200 MORE gallons of fuel at $5 each, costing more like $1000 more in fuel each week, each truck. That's a loss of $1.04 MILLION a year. :shock:

I'm not sure about Wind.... but I am not talking about going 5 mph slower in the SAME gear at lower RPM's. I'm talking about how limiting my top speed, has caused me to spend MORE time in a lower gear at higher RPM's.

 

the mega carriers are going to start specing the new trucks to be able to run at 60 mph and get the best mileage. you guys are right, simply lowering the speed to 60 on a truck that was geared to run 65-70 isnt going to do anything but piss off the driver, it will probably make him use more fuel and it will certainly decrease the life of the engine.

 

All trucks have a "Sweet spot" for MPG.
For most trucks it is somewhere between 60 and 70.
Slowing down to under 70 will save you fuel it's not the trucking companies that say this it's the manufacturers. If they could sell a truck with better fuel mileage over 70 don't you think they would build it for all the people that want to run over 70.
Under 70 you need to figure out where that sweet sopt is.
Everyones driving conditions are different but if you are running OTR you'll have your mix of flat and hills an average of driving conditions.

 

Yeah, and they also say that if you want fuel economy, STAY AWAY FROM A CAT. But, back in '00, I had a FRT Classic with a 3406 at 500hp that was geting 7.6 to 8 mph for an overall overage, and it was not governed. But, it did NOT have 3:73 or 3:55 rears. It had 3:43 rears, and running the same roads as Hobo, that was the average. I have NEVER been able to match that with a governed truck. And, we DID try setting the cruise at different, lower speeds because WE were paying for the fuel. Staying up in the 70s got us the best economy. Not lower. And, it was the UP-GRADES that KILLED us on economy.

Climbing a hill, you're GOING TO HAVE YOUR FOOT TO THE FLOOR, no matter what gear you are in. In 10th gear, you're using about 4.2 mpg. In 7th gear, you're using DOUBLE THAT to go the same distance. But, you DO NOT make that up in the short time you coast down the other side. Going up a hill, you use more than double the fuel that you save running on level ground.

The concept looks good on paper, but falls very short when put into practice.