"Secrets of Better Fuel Economy - The Physics of MPG"

http://www.cummins.com/na/pdf/en/pro..._Whitepape.pdf

A Cummins guide for the big rigs. Most of the discussion also applies to our towing with light trucks.

 

You want better economy? Drive slower. My best mileage 18.8 on a recent trip was done at or slightly below 65 mph, intermixed driving hills, curves and flats.

I'm surprised some states haven't dropped their speed limits back to the 55/65 of olden days. One thing is certain, the slower you go the better the mileage, at least with CTD's. I once got 24 mpg @ 52 mph.

The wider tires don't help. Stick with what the mfgr put on to begin with.

 

Yeah but what they don't tell you is that lowering the speed limit from 70 to 55 increases the traffic on the highway by about 25% You don't save much fuel sitting in stop and go traffic.

 

I was on/off the interstates over the weekend (1200 mi trip). I saw no difference in the amount of waiting time, getting on/off either highways. Not that much traffic on Friday (daytime) and yesterday (Sunday afternnoon). I actually did better time wise taking the older US and county roads vs the Interstates.

 

When traffic is light it doesn't matter. When it's busy...70 mph does handle more cars.

27 Shammy with Cummins....MMMMMMMMmmmmmm. Lemmee seee...lemmee seeee

I'm looking at a gasser 20' pilothouse right now.

 

"Yeah but what they don't tell you is that lowering the speed limit from 70 to 55 increases the traffic on the highway by about 25% You don't save much fuel sitting in stop and go traffic."

Ya know, I just don't get that.

 

Ok, if you have to drive 70 miles and the speed limit is 70 mph, you will be on the road for one hour. Now drop the limit to 55, after an hour on the road you have driven 55 miles. 21.42% more time on the highway for the same miles.

Truckers make that much less profit for miles driven, this loss is passed on to the consumer in the form of inflation. Go ahead guys, let the EPA distroy our economy. (some more that is)

 

SWIFT and Wally World trucks are already doing 65 and it doesn't seem to be affecting their deliveries. O'course SWIFT stands for Slowly Weaving in Fast Traffic.

The argument about saving time is nonsense. The difference 70 vs 65 is less than five minutes vs 55 which is more than five minutes. I used to have a comparison chart about this. It was hardly worth the effort to drive that fast. I once figured out that for every five miles over 55 I lost six miles per gallon.

 

That is good stuff. Interesting statement that some drivers can get 30% better economy by their driving techniques.

The aerodynamics of pulling a high load really plays into speed versus fuel economy. I do one (or better) mpg better at 60 than at 70 pulling the fiver. I do another .5 mpg doing 55. These are tested using the computer for short sections about 10 miles each trying different speeds. Computer reads about 10.9 at 70 - - reads 12.7 at 55.

I am heading out for about 12,000 miles pulling the monster. I will be thinking about it all the way, and especially when I am bleeding at the fuel pumps.

 

Yes it is true for my truck that I do get better MPG at slower speeds, but as I found out this past weekend running 55mph does not sit well with my auto tranny pulling my loaded TT. It's too close to the shift points and out of the torq power band.

I like the fuel milage empty but pulling, I would rather be in the power.

 

I was on a 70 mph road this last weekend, going 20 mph in heavy traffic. Speed limit didn't seem to affect it at all. What gets me is in an area where I am paying $2.25/gal I am still the slowest on the road. Most everyone else is going 75 in guzzler SUV's, but they are about the cost of fuel.

 

A 700 mile trip at 70 mph would take 10 hours. In the same ten hours at 55 mph you would only drive 550 miles. That's not significant?

Slow down to 45 and you will have gas running out of the tank Big truck only gets six miles a gallon on a good day. On my truck, doing only 50 will give a mileage help of about 1 1/2 mpg as compared to 75. About the best I can do is just under 70, like you see most trucks driving, about 68.

If you are an independant, fuel is not necessarily the most cost driven consideration. Time usually wins out. Several hours difference to California generally means a day sooner for the reload and therefore much more money to pay the bills.

You said you don't see a difference, how bout this.... A driver can log a maximum of 770 miles in an 11 hour day. With the 55 he can only log 605 miles in the same eleven hour day. Here's the real numbers, within five days he gets another full day driving in at 70 mph. You don't think that affects rates?

If a driver drives 100,000 miles in a year, he is on the road at 55 mph, 390 hours more than at 70. Thats 7 1/2 hours a week more time spent on the highway, conjesting traffic. Thats over 35 more sleep times that must be taken with the fuel burned, expenses for stops and meals. Sounds like it is about a month of work lost per year.

"The argument about saving time is nonsense" yep, but only because it is not your money, but in the end it is, you just don't know it.

 

Wow thanks for the school lesson!!!!!

Anyone wanna refute that?!

 

Yes our trucks get better milage at lower speeds, but most CARS are designed to get better milage at a higher speed.
I remember when the national speed limit was lowered to 55 back in the 70's.
At that time, I was driving a datsun 240Z, which was geared VERY tall, and got it's best milage at 75MPH. (wish I still had that 240Z)

 

The example given to address driving at a slower speed is only applicable to those who depend upon driving for a living.

"If you are an independant, fuel is not necessarily the most cost driven consideration. Time usually wins out. Several hours difference to California generally means a day sooner for the reload and therefore much more money to pay the bills."

I agree that the lost time for an OTR driver will be cost prohibitive after taking into account the above stated issues, however for the daily driver speed/time is less of a factor and it becomes more economical for them to drive at a slower speed to save fuel and still make their money.

So the time vs speed arguement is dependent upon the ultimate needs of the driver not the time/speed variables.

-CM