Does slowing down make a difference? If so how much?

[Gene Arnold]

I just got home today from a 3464 miles, five weeks trip to Nebraska and Colorado to visit family. We debated about the advisability of going due to fuel costs but the "clock is ticking" and someday in the near future I'll not be able to do these trips (I'm 73 next month).



I do try to be more careful with my speed, more intelligent with my planning, etc. When we started, I determined to hold my speed to 2000 rpm (57 mph). I think it paid off. For the entire trip (a lot of miles in the mountains) I used 209. 6 gallons of diesel at a cost of $913. 10, which is 26. 4 cents per mile.



My best towing mpg was the leg from Laramie, Wyoming to Grand Island, Nebraska; 20 gallons even for 412 miles, or 17. 9 mpg. Lowest mpg was Boise to Cokeville, Wyoming-- 355 miles on 23. 3, or 15. 2 mpg. I paid a low of $4. 00 on April 22, and a high of $4. 90:{ on May 24 at Craig, Colorado.



We're already debating about whether or not to go for our normal fall trip to see the kids. We'll watch the trends, but as I said, the clock is ticking on this old man! I don't want to be sitting in my dottage with regrets! I haven't compared my total cost of camp fees, groceries, etc. with motel and restaurant costs. I know I still save a considerable amount, plus I prefer my wife's cooking and the privacy of my Airstream.



Gene

 

[glhs]

My best towing mpg was the leg from Laramie, Wyoming to Grand Island, Nebraska; 20 gallons even for 412 miles, or 17. 9 mpg. Lowest mpg was Boise to Cokeville, Wyoming-- 355 miles on 23. 3, or 15. 2 mpg.



Gene

Is it me or does 412 divide by 20 = 20. 6 mpg?

I bet the first 10 miles east of Laramie didn't help!

 

[McCRam]

Dropping my speed 75 to 65 gives me 20% better in the cummins. Two years ago I got 25 mpg going 50 mph on icy roads. My car a 2005 Grand Prix is 10% better. As speeds star to climb above 50, aerodynamics starts to play a huge part in fuel economy. If you are driving into a strong headwind, it pays to slow even more.

 

[MMichaels]

Slowing Down To Save Fuel

I sometimes drive from Down Here (Scottsdale) to Up There (Flagstaff) and by reducing speed from about 68 to 60, find that I'm getting roughly 3 miles per gallon better mileage - this while pulling a Featherlite 3-horse trailer...



I just sit back, watch the scenery (and the semi's) roll by, and, occasionally, take a nice nap.



And I'm sure I'll get even better mileage if I reverse the set-up and attach the horses to the FRONT of the truck...

 

[Gene Arnold]

Is it me or does 412 divide by 20 = 20. 6 mpg? I bet the first 10 miles east of Laramie didn't help!

Thanks for catching this. My old eyes jumped a line. It was 23 gallons.

 

[JMetzger]

I know that the last time I pulled the RV I drove 55 MPH & got 16 MPG. When I drove 63 MPH I got 14 MPG. These were mostly on level ground. Going slower does make a difference for me anyway.

 

[TriSum]

For 8 years I had access to a computerized dyno for testing vehicles... I guess that I have over 400-500 hours doing so... that was 30 years ago... but the math still applies...



The math... . and its nothing more than an engineering formula goes something like this... for every 2-3 mph over 55 you lose 2% in fuel economy because of the following fixed numbers... .



Weight, wind resistance, rolling resistance & acceleration to speed. As the speed increases the wind resistance quadruples every time you add 10 mph in speed...



Just my thoughts on this...

Jelag, or anyone else,



After reading the article from Cummins. They state the efficiency range of the engine is at a higher RPM than most of us choose to drive at, (2000K +). At what point does MPH (wind resistance) lose or gain versus keeping the engine in it's recommended power band?



IE: I tow very heavy, (20K+), when I drive 55 mph I'm at about 1500 ish RPM. The truck will kick out of OD at every hill. If I drive 61 mph I'm at a little over 1800 ish and it doesn't kick down nearly as much, only the BIG hils.



Now, does keeping the engine in it's RPM band = being more efficient and therefore overcoming the additional wind resistance ?????? Or is it still better to keep the resistance down to obtain the best MPG ???????



David

 

[georgej]

Just drove to Virginia from Oregon pulling my 12500 lb fifth wheel . The first 1500 miles I kept the speed maxed at 58mph and was dissapointed with the mileage plus the engine seemed out of it's power band. At every hill I had to shift out of OD to keep up the speed. For the rest of the trip I drove between 62-65 mph and saw no difference in mileage and because the engine was in it's powerband it pulled many hills in overdrive. I averaged 11. 4 mpg for the 5084 mile trip to Wise,VA. Now I need to save up for a couple of months for the trip home:{



By the way I saw fuel prices from 4. 17 to 4. 84 per gallon!

 

[TriSum]

That is kind of what I'm seeing. If I stay at 1800-1900rpm the truck seems to pull "easier" not as much gear hunting.



I'll do some more testing. My problem is my load weight changes every few days.



David

 

[E7mack96]

I have gotten near 25 (24. 87) mpgs doing 55 from oh to FL and back. I have also gotten 18. 5 mpgs doing the same run at 70 mph.



When I pull my little trailer I get 15 mpgs at 70 on this run. I once did 80 and only got 13 mpgs pulling the trailer. (BTW, this trailer was empty and only weighs about 1200 lbs if that. )



The wind resistance is what kills us.



What sucks is i have to drive 850 miles one way, and at 55 it takes f***in forever (over 15 hrs). At 80, it only takes 10. 5 hrs. At 70, it takes right at 12 hrs.



Whats your time worth? For 3 more hours at home, i'll drive faster and eat the cost.

 

[CumminSense]

P₀ = P + 1/2ρ v²



that is



stagnation pressure = static pressure plus one half air density times velocity squared



stagnation pressure is the pressure at the "dead air" spot on the leading edge of a body in motion, as at the bumper (usually applied to an aircraft wing, but it essentially applies to road vehicles also).



So drag is proportional to the *square* of the velocity -- not 50% more, not 10% more, not cubed. So double the speed, 4 times the drag. Triple the speed, 9 times the drag (3x3=9).



going 70 mph vs. 60 mph is 16. 7% faster, so the drag will be:



1. 167 x 1. 167 = 1. 36 = 36% more drag



Now the fuel usage won't be exactly 36% more, as engine efficiency, rolling resistance, and air flow dynamics around the vehicle body come into play.

 

[Gary - K7GLD]

Just drove to Virginia from Oregon pulling my 12500 lb fifth wheel . The first 1500 miles I kept the speed maxed at 58mph and was dissapointed with the mileage plus the engine seemed out of it's power band. At every hill I had to shift out of OD to keep up the speed. For the rest of the trip I drove between 62-65 mph and saw no difference in mileage and because the engine was in it's powerband it pulled many hills in overdrive. I averaged 11. 4 mpg for the 5084 mile trip to Wise,VA. Now I need to save up for a couple of months for the trip home:{



By the way I saw fuel prices from 4. 17 to 4. 84 per gallon!

YUP - just got home on the return RV trip from Stockton area of California. I usually see 16+ MPG towing our 5er, while traveling right at 60 MPH - the engine RPM is about 1700 RPM or so, and all is happy. But this trip, we followed my brother-in-law driving his SLOOoooooow Ford V10 powered Mini-Winnie, and we ran mostly at just under 55 MPH - maybe good for HIS MPG, but dropped mine a good 1-2 MPG. My engine was nearly lugging down at about 1400 RPM, but shifting into direct wasn't really an option.



Too bad, I really wanted to do another MPG test run. but was too far down on power band and efficiency for a proper result...



OH, yeah - diesel in Alturas, N. Cal, was $5. 36 a gallon... #@$%!