MPH And Fuel Saving...
 

Is there a place to go, or could someone tell me if there is a real difference in how fast you drive and the fuel saving. I was told by an o/o that he saves hundreds of dollars per week in fuel because he only drives 60 mph as opposed to 70-72 mph. He said he showed and compared numbers with a friend who drives fast and the saving was a big difference. He has a different tractor then his friend but the loads they haul are the same weight.

He told me to fill up and drive a tank full out at 60 mph then check it. What is your opinion?

Thanks
Neckster

It depends on how the truck is spec'd. My truck gets better fuel mileage at higher speed, higher rpm than it does chugging along. I can chug down the highway at 5.3-5.4 at 60mph in 13th gear and turn 1300 rpm, or I can step it up a bit to 70 and turn 1500rpm and get 5.8-6.0 depending on terrain, etc. of course. I've heard that the newer trucks can run lower RPM without sacrificing fuel mileage, don't quote me on that though.

Last night my computer showed a 1% savings by slowing from 65 to 55. That's about $20 a day.

I have a Cat C-12 and run around 68-72 mph and get 6.3 mpg. I don't know if that's good or not. I've never tried going 60 mph, that's slow, however I did try 63 mph and that was not to bad.

Neckster

An example would be the CAT C-15 ACERT engines. They shouldn't be cruised at more than 1500 rpm.

Neckster- As said before, the newer engines have been rumored to benefit from lower RPM and speed, so it may work the best for you. I can't stand driving that slowly either, but I do know that I tried it one night, dropped the cruise down to 63 or something in 12th gear and cruised and when I got to where I was going, I wasn't as tired or stressed out feeling, as I would have been from doing 70-75. Sounds goofy, but just my findings.

PackRat-I told you not to quote me, dangit! :lol:

The "sweet spot", for my 3406E was at 1400rpm, for my Acert, it's about 1325rpm.
But there is more to it, than just RPM, the same RPM, sure use a different amount of fuel, at 20, or 40psi boost.

And nobody could change the law of phythics, higher speeds=higher air resistance=more energy=more fuel, something like that! :)

And the less airodynimic truck consumes more fuel, for the same reasons.

I'm not entirely sold on the RPM differences. I've tried it out several times. Cruising in 12th gear vs 13th gear. At 60 mph, that's a difference of 1400 vs 1600 rpms.

According to the computer, there was just no difference at cruising speed. However, climbing a hill, because the engine is in the higher RPM range, it will inject a bit more fuel. The 13th gear will just slow down a bit, while the 12th gear just powers along. I think that's where most of the difference is.

There is a much bigger difference in speed though. Solo is right, as you increase speed, wind resistance goes up exponentially. The horsepower you use to push through this air is "wasted" just to keep your current speed.

I should edit this. I meant to type 10%.

And in the interest of full disclosure, mileage went from 9.25 at 65 mph to 10.03 at 55 mph bobtailing with a T6 KW/N14 Cummins 410hp, 3.90 rear gear.

I had always thought the computer on this truck was set up for US gallons, but now I'm sure. I can't imagine I was getting 10 mpUSg.

And why not? I was getting close to 11mpg, cruising bobtail at about 64mph.
Not a computer #, actual, at the pump.
C-15 CAT in T600 KW. :D

I have gotten around 10 mpg bobtailing. Unfortunately, I haven't found anyone who will pay me to bobtail. :lol:

It is, metric is in L/100km, not imperial gallons.

I get 12 mpg bobtailing sometimes.

OK, Hopefully I don't mess up the math here.

100,000 miles at 5mpg with fuel at $2.60 =$52000.00
100,000 miles at 6mpg with fuel at $2.60 =$43333.33

One more mpg is worth $8666.66 more money for you at the end of the year. And as fuel prices go up that number does as well. And that's alot of money to me.

I for one have much better fuel economy when I drive a bit slower, 65 or under, never over the limit. And like someone who does not like to be quoted said, I arrive calmer and less stressed as well.

And for the fun of it lets dive a little deeper into the math. Lets say you have 1200 miles to cover.
Run it all at 75mph and it will take you 16 hours.
Run it all at 65mph and it will take you 18 hours 28 minutes.
As those runs get shorter, the time "savings" become even smaller. Slower speeds frequently result in better economy for me.

Many a wise driver will tell you bad stuff happens fast, the slower you go, the more time you have to react. Bad stuff could be a collision or a ticket. Or it could be you running faster than everyone and changing lanes alot, increasing your risk for a collision. Have a collision or get a ticket the fines are the least of your worries. Wait until your insurance finds out.

So..................

Run slower and you make more money and are less stressed. Relax and enjoy yourself!

Now, some will say that if you drive slower you might miss that next load. I would counter by saying I would have to miss alot of those "next loads" to equal my savings in fuel alone each year.

10-4 on that!!!!

I took marylandkw's numbers one step further....interesting that it is not a linear change when you slow down and bump up your fuel mileage.

I took his(hers?) numbers and went one step further and figured the cost per mile for different mpg's:
5.0mpg=$0.52/mi
5.5mpg=$0.47/mi (0.05 difference from 5.0mpg)
6.0mpg=$0.43/mi (0.04 diff from 5.5)
6.5mpg=$0.40/mi (0.03 diff from 6.0)

What is interesting is that there is an increase in 1 cent per mile savings for each 1/2 mpg increase between 5.0-6.5. So depending on where you are with your own mileage, you can save anywhere from $3,000 to $5,000 per year. That's not chump change!

Last time I checked I am a "his"

The difference off of your calculations from 5mpg to 6.5mpg at 100,000 miles a year is $12,000. I don't care how good your company is going, $12,000 is some nice extra money.

Keep in mind, those 12 grand, is not just "gross", it's "net", meaning, goes directly in your pocket, no overhead! 8)

Slow driving doesn't equel good mileage. My truck is limited to 65 mph, and recently the management turned it down to ... 62.5 mph both on foot and cruise because they want at least 6.5 mpg. Of course they buy cheap trucks (Columbia) with cheap Mercedes engines (1550 lb-ft of torque) and they send us to hilly terrain in PA, NY or BC with 45,000 lb loads.

Mercedes 12.8 l 460 hp 1550 lb-ft of torque 3:70 axle ratio
Limited (for now) to 62.5 mph :(

Sorry marylandkw...that avatar was a bit androgynous.... :)

60363...as a driver and not an O/O or carrier, you do not see the benefits. Slowing you down puts cash into the owner's pockets but not yours... :cry:

My post was more addressed to carriers and O/O's. And solo has a great point. That $12K is net and it comes right off the bottom line. If you earned $0.50 per mile profit, you would have to drive 24,000 more miles in a year just get that $12K back.

Dang that sure is a good incentive to inject some helium in the right foot to get it a little lighter... :D

Do you think the on board computer readout of your MPG is accurate? I'm wondering if it's good enough to play around with different speeds and gears when on a flat highway?

With my old truck(3406E CAT), my ECM numbers, and "at the pump" were about 0.7-1mpg different. ECM showed better mpg.
My current Acert ECM, and "at the pump", is just about the same.

Anyway, i watch my mpg every fill up, "at the pump"!

From a recent article on fuel economy in the Canadian trucking magazine, "Highway Star" ("There's No Silver Bullet", by Jim Park)

"...Jim Booth ... is a former test pilot for Caterpillar, and a zealot when it comes to fuel economy. He's retired from test service with Cat, and now hauls freight for Cat between northern Wisconsin and Peoria, IL. He manages 9.3 mpg consistently our of a C13 Cat.

His style is to step so easily on the pedal that you'd swear it was made of crystal. Most would call his accelaration painfully slow, and his adherence to 55 mph nearly unbearable, but you can't argue with 9.3 mpg."

I hate to be a fly in the ointment...but as a stickler for technical accuracy...
wind resistance does not go up "exponentially"...
wind resistance is proportional to the square of the velocity...

simply put...double your speed=> four times the wind resistance...

so...55mph vs. 70 mph will yield a 61% felt increase in wind resistance.

Still substantial and worthy of consideration...but not nearly "exponential".

IIRC, anything "to the power of two" or "squared" is an exponent.

Columbia is not a bad truck, i own one with Detroit 475, dont let the lower price fool you, thats a good truck that a'lot of my friends have had for years...

And who said a higher education doesn't help in trucking.....Watch out, the techies are taking over!!!

What you said right there, is what exponential means.

I don't know what "IIRC" means.....but:

to the power of two means it varies as the square of the variable...
the two is the exponent and it is fixed....it is only correct to say it varies with the square...(it could be cube or fourth or fifth power etc but the exponent is fixed......)

in order to vary "exponentially" the varible would have to be the exponent with a fixed (or variable) base.....

e.g.: IF resistance 'R' varied "exponentially" with velocity 'v' and constant 'K' as the base the equation might be:

R=K to the power v (I do not know how to express a superscript here)

thus...if you double the v
R=K to the power 2v and you have effectively SQUARED the R
BUT>>>>if you triple the v
R=K to the power 3v and you have effectively CUBED the R
and so on with greater increases....

an "exponential" function increases at a much greater rate than a simple square function....
but do not lose the salient point here.....a simple increase in speed does not just give you an equivalent increase in wind resistance (and presumable decrease in fuel economy)....it goes up much more quickly!!

I do not want to be argumentative (even though I am) but specificity and accuracy are important...

Here endeth the lesson.... (until next time!!!)

If I remember Correctly, it means, If I remember correctly. :D

Yet again, another reason to lighten up on that right foot!

I beg to differ. Not in this case. I'm sure the original statement was understood by all. :roll:

Dejanh,

I have nothing against FTL Columbia ... the purpose of my post was to demonstrate that when the truck is not spec'ed right for the job (as is the case with my Columbia), the fuel mileage will be poor no matter how slow you drive. The fact that my boss further turned down the limiter to 62.5 mph (both on foot and cruise) means they were not even getting 6.5 mpg when we were allowed to do 65.

44,000 lb loads in hilly terrain can do that to you, no doubt.

Columbia 2006
Mercedes 450 hp @ 1900 rpm, 1550 lb-ft torque @ 1200 rpm
3:70 rears, 10 speed autoshift
Frequent visitor to: PA, NY, KY, TN, NC, British Columbia, Alberta :(

How come in this "saving money -rate of speed" discussion no one ever mentions that a truck may hold up longer all around if driven slower?

Cause the point of discussion, was "Fuel savings"? :P
But of course you are right, on that one! 8)

Not to make this a religious discussion but, AMEN! :D