Some simple math regarding fuel economy

The OP is correct in the thought process of driving slower saves $. HOWEVER... those that claim to get same or better mpg driving faster clearly have an issue with their truck. If you have 2 properly operating trucks, then there should be a 1 mpg difference in 65 v 75 mph. Although in some cases where a driver avg 2 to 3$ a mile with the national avg fuel surcharge rate, then there is no incentive to slow down. August 20th will be my 5th month as an o/o and I have avg 8.37 mpg per month. This month I'm trying to break 9. I have an avg of 8.87 and that is driving 85% @ 60mph. 63% is HWY miles and rest is CITY/IDLE.

Take my post with a grain of salt. I know I have lots to learn about trucking but I know the math and numbers. If it was anything I excelled in when I studied mechanical engineering, it was physics and dynamics.

 

How do you come up with $2000?

$2000 buys about 500 gallons today. If you burn 1 gallon per hour you have 500 hours of idle time per month (30 days) which then means you must idle for 16.6 hours per day to spend this kind of money. If your truck only burns 0.5 gallons per hour, you now have to idle for 1000 hours per month, or 33.3 hours per day ..............................

 

My truck averages 1 GPH idling. Let's say you ran a perfect month, with ideal dispatches, and drove 10 hours per day continuously for 30 days. That leaves 14 hours per day idling, the way some truckers seem to be doing it. 14 hours x 30 days = 480 hours = 480 gallons x $4 / gallon = $1920 / month. Of course actually maximizing your 70 every time isn't happening in the real world, so that could easily be more... a lot more idling time.

While some folks claim to have 0.5 GPH in their truck I wouldn't know about that. I've tracked the GPH in a 2012 Cascadia and a 2003 KW, and both were right around 1 GPH. If you are getting 0.5 GPH then you could use a rough figure of $1000 / month for idling if you run a perfect month to maximizing your 70 all the time. Take any 34 while idling in there or run less than your 70 and it will start pumping up that number in a hurry.

Whether the number is $800, $1000, or $2000 per month I would squeeze bricks out my anal orifice if I had a heating or cooling bill like that in my house. The thing is I don't think most folks that are claiming they "just can't sleep unless I have a cool area" have ever tried to sleep in anything else for an extended period of days. Of course if you have a habit of always sleeping in an air conditioned cocoon, then getting out of that "comfort zone" will be unusual at first. After a few nights I bet you will find you can get just as good and maybe even better sleep by having some fresh air.

What do you have do lose by trying a little non-idling experiment? What do you have to gain? I can certainly use $12,000 to $24,000 more per year on my bottom line when I become an O/O.

 

I'll idle all night if it's below 15 or above 90. I have an Espar heater for the winter and a turbo fan for the summer. I agree on the humidity, idle no matter what if it's humid. I can't deal with high humidity.

 

First, I understand this post is directed towards o/o's.

I am a company driver for Prime. While saying this some of you may want to kill me.. All I can say is ok... lol. Seriously though, I'm driving a new 2014 freight shaker cascadia evo, with a DD15.. I'm not sure the horse power or gear ratio. I cruise at 62 all day and average around 7.5 to 8 mpg. Loads, winds, and weight included. My first truck I drove was a 2010 international with an ISM engine that got the same mpg as this cascadia but pulled hills twice as well. The only easy difference was the international DIDN'T HAVE DEF. I don't idle over night or at shippers and receivers. Prime I know cuts down the hp in their trucks and uses a highway gear ratio. That alone should not boost the fuel economy alone.

I'm still what you guys would consider a baby in trucking. Under 200,000 miles..

 

Mega companies buy cut down trucks with small motors and govern them down, to save money on fuel is correct, But, they are throw away trucks, witch is why they trade them in after 3 years, for an O/O, this is not the same, Someone that owns 1 truck needs to look at length of ownership of truck, the 400 HP motor will work fine for OTR, mountain climbing, heavy loads, but it will take its toll on it quickly, Now on a 550 HP power motor, will work less driving in the same conditions, Mountain climbing, heavy loads, The 550 will pull hills with much ease than the 400, Know I'm not saying that the drive with the 550 should drive faster, but the 62 MPH is a myth, 72-75 with proper motor size, Axles, tranny will al come in play, if the truck is set up properly, a driver with a 550 motor, will out last the driver with the 400

 

I've never understood this "will work less" argument. It is making more horsepower, therefore it is working harder. Horsepower is the rate at which work is done.

I don't understand your mention of 62 mph is a myth either. There's no magical spec that will allow you to get the same MPG at 70+ mph as at ~60 mph. It simply doesn't happen. It's all truck driver myth perpetuated by salesmen. Oh yeah driver if you spec your truck correctly you can get the same MPG at 70 mph. Going faster requires more horsepower to maintain speed and more horsepower means more fuel burned. It really is that simple.

All of the data I've collected over the years in multiple trucks says no. And I've collected a *LOT* of data.

 

IMHO the "myth" is that you can get "just as good fuel mileage" pushing air out of the way of your truck at 70-75 mph as you can at 62 mph. To borrow a famous political phrase, "It's the wind resistance, stupid." The energy required to overcome wind resistance increases exponentially with increased speed. While it is true that a truck with greater horsepower will be overcoming that increased wind resistance with less than 100% of available power, the fact is that the amount of power required to push against the air doesn't change at all.

Engine efficiency is one thing, but we are all up against the wind. More efficient aerodynamics will certainly be helpful, but even then the most efficient set up for speed, gearing, and horsepower will most likely be in the 62 mph range.

 

Aerodynamic drag squares as you increase speed. So........

The drag at 75 MPH is 56% greater than at 60 MPH.

60= 1

75/60= 1.25

1.25x1.25= 1.56

75= 1.56

No amount of gearing, "set up" or slope on a hood will change that fact.

If one should drive 75 instead of 60 is another question entirely.

 

If anyone has doubts about the speed vs fuel consumption ratio dand you're paying the fuel bill do yourself a favor and google 'cummins the physics of mpg'. you will see the fuel consumption is exponentially higher as you increase your speed. tyats unfortunately how it works. if youre a company driver with no MPG incentives then hammer down. Regardless of who or how you operate it woudnt be a bad idea to at least browse through the cummins whitepaper. you will likely be quite surprised by some of the things you find such as what impact different road surfaces have on mpg. that is not a variable you can control but speed certainly is.