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.