My 55 mph test had to end this week due to schedule obligations (too many miles with not enough time). First, let me begin by saying “IT WORKS”. Traveling at 55 mph increased my fuel mileage. Previous to the test, I was running 65 mph and averaging 9.5 mpg. The test lasted 4 tankfuls and I averaged 10.528 mpg.

The above resulted in a mile per gallon increase. However, let’s not get too excited and I’ll tell you why. One of the fill up’s came in at 10.78 and this was one of those trips where all the stars aligned just right in regards to traffic, terrain and weather. I did not have a tailwind. By comparison, when ending the test after three weeks, I was 580 miles into the tankful still cruising at 55 mph when scheduling required me to step it up to 65 mph. I traveled the next 1,194 miles at 65 and the combined fill up came in at 10.482 mpg. This isn’t bad for basically being at 65 for 2/3 of the tankful.

What does all of this mean in regards to dollars? I’m going to give you some figures based on my regular run from Charlotte, NC to Laredo, TX. The round trip is 2,885 miles and we’ll use a fuel price of \$3.709. At 9.5 mpg, the truck would consume 303.684 Gallons @ 3.709 per gallon = \$1,126.36. The same trip at 10.5 mpg would consume 274.761 gallons of fuel @ 3.709 per gallon = \$1,019.09 for a saving of \$107.27.

Now… let’s talk about what this means in regards to time. For the total trip, my average speed was reduced by 7 to 8 mph. This would result in an additional 6.59 hours to complete my normal 2,885 mile round trip. We saved \$107.27 for putting in an additional 6.59 hours which equals \$16.27 per hour. I guess this number is fine as long as you’re not missing out on family time, load opportunities or personal time. At the 55 mph speed, with these results, if you missed a load or ended up sitting all weekend because you missed getting unloaded/reloaded on a Friday, this surely isn’t worth this amount of money. If you went from 6.5to 7.5 mpg on 2,885 miles using the above figures, the result would be a savings of \$219.51. This is a significant amount of savings as compared to going from 9.5 to 10.5. In this case the additional 6.59 hours would be worth \$33.30 an hour. This might make it more worthwhile for your extra time. However, you still wouldn’t want it to cost you a load that would leave you sit for the weekend.

Overall this 55 mph test opened my eyes to many factors. First, the fact that I could drive 55 mph and it wouldn’t really affect my operation all that much on most days. I was also surprised that on some days, it hardly affected my arrival time at all. The Detroit DT12 Transmission really helped to keep my average speed higher than I expected with a cruise setting at 55 mph. This was primarily due to the ECoast function of the transmission. What happens with ECoast is each time I’m on a downgrade; the truck would roll well beyond the 55 mph cruise setting. It really requires me to think of my cruise setting differently. Essentially, you’re only fueling/powering the truck to 55 mph. It’s amazing when in a 70 mph speed zone, how often you’ll be coasting at 70 mph. I find it similar to a roller coaster in that there were times that I went for several miles with the engine simply idling and coasting. I also found that while in Texas on flat ground with little coasting opportunities, I would lose 100 plus miles of productivity per day. This leads me to my next test which has been inspired by some of the really high one day numbers (especially the 12.80 for 420 miles at 68,000lbs) which is to operate at 57 mph which will keep me away from my shift point in the rolling hills sections.

The one issue I experienced in South Carolina on I-85 was each time the truck rolled out of ECoast and hit the next upgrade; it would instantaneously shift into 11th gear and quickly shift back to 12th. This shifting was due to the fact the transition was being performed at a natural shift point for the transmission. This week, on my way home, I experiemented in the 60 mile an hour zone by trimming it back to 57 mph and found the downshifting scenario was almost completely eliminated.

In the upcoming weeks, I plan to run 57-58 mph from Charlotte, NC to Picayune. I’ll continue on to Laredo, TX and back to Picayune. The future will tell us if operating at 57-58 mph for the northern half of my route and 65 for the southern half may combine the best of both worlds in relation to fuel efficiency and time management.

March 31, 2017

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Good article. Most of the time I can travel 55 mph to get the greatest profit out of a trip. But as an OO my truck is set wide open so when I need to spend the extra fuel dollars to get the load delivered within my hours, I have that ability. A recent run I had to run the speed limit, 75 mph, and I lost 2 full mpg. That cost me an extra \$100 in fuel alone on the run. What most people don't take into account, maintenance and tire expenses go up at faster speeds. That cost is harder to calculate, so most don't take that into account.

May 29, 2013 18:52:34 PM

All of the big fleets always promote the numbers for better fuel mileage and the reduced costs of going slower. Thank you for putting in the driver's time as a factor, no one else seems to think along those lines. I wish trucking companies could say what a hour of driver's time is worth to them (other than zilch) and perhaps more importantly what is a log book hour worth, as far as revenue and productivity.
The other way to look at this, that I have not seen done, is to figure out how many total miles for revenue purposes you want to run. Then calculate the differences between a faster and slower average speed. You may be surprised that it can take 2 more weeks worth of hours to run the same total miles at a slower speed, over an entire year. What are those 2 weeks worth to you, or what will you give up, either in revenue or personal time, to create them?

May 23, 2013 10:47:30 AM

About 90% of the time I run between 57-60 mph following Kevin Rutherfords line of thinking. There is definitely a big difference in my fuel mileage by doing this. I have noticed, with my older truck, that my oil consumption is down when running slower, too. The other big advantage, to me at least, is that it is SO much more peaceful and relaxing during the day running slower because I don't have to worry about changing lanes to pass slower vehicles all day and getting upset when I can't change lanes due to heavy traffic. I may have fewer runs during the year, but that is the good thing about not having a truck payment.

May 23, 2013 6:08:23 AM

Interesting. Thanks for running the numbers too as that's what it's all about. Did you have a reflective triangle on your trailer so none of those chicken haulers would rear end you?

May 21, 2013 19:26:31 PM

I knew slower was better less work for the engine and transmission which results in better fuel efficiency

May 21, 2013 18:36:16 PM

I like your article, you've put quite a bit of thought into the many variables. I'm driving a std 10 speed & am finding out that weight is the major factor, so if i'm light the speed doesn't affect my mpg very much, but when I'm heavy, I slow down to 58/60mph, shift at a lower rpm, & take full advantage of gravity downhill. I'm seeing an improvement so I'll keep working at it. Thanks for sharing!

May 21, 2013 17:55:39 PM

Great Job Henry! I agree with both of you it is a matter of balance with the goal to make the most money and have the most money in our pockets at the end of the day. We have to level the teeter totter as Henry says between expenses and revenue.

May 21, 2013 5:42:07 AM

I have had some success experimenting with 60 mph. My first 450 miles on the tank while loaded light-about 7,000 lbs on one load and 6,500 lbs on another averaged 9.51 mpg. It picking up 2 loads, delivering 1 Chicago rush hour. That was with a non aero trailer. But, the slower speed did boost my MPG. I will definitely lower my speed-when I am not conserving hours. With an EOBR in the truck, my tendencty is to start the week at 65. If I see that I can slow down-I will.

May 21, 2013 4:17:27 AM

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