Most fuel efficient highway speed?

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Most fuel efficient highway speed?

May 18, 2013, 11:22:13 am

Has anyone noticed a difference in fuel efficiency between driving 55 mph and driving 60 mph on the highway? I realize that driving faster than 60 mph causes a decrease in fuel efficiency. I was wondering if driving 55 mph causes any increase? My unscientific test results have been inconclusive so far. I seem to be getting 10 mpg at either speed but possibly more transmission wear from more shifting on the hills at 55 mph.

Scott

Re: Most fuel efficient highway speed?

Reply #1 – May 18, 2013, 11:28:21 am

In rolling hills, no wind and 55mph, I've often got 12mpg and even more. All across Upper Michigan one time (east to west). Most recently all the way from El Centro to Las Vegas.

Still more at 50mph.

Rarely get 12mpg at 60mph

best, paul

Quote

Has anyone noticed a difference in fuel efficiency between driving 55 mph and driving 60 mph on the highway?

Re: Most fuel efficient highway speed?

Reply #2 – May 18, 2013, 11:49:17 am

I agree with Paul's results. Last fall while traveling South through Kansas (arrow straight road and nearly level) I ran a test. One hour at 65 mph in 6th, and one hour a 55 in 5th. All results were taken from VMSpc. 6th gear results were 8.9 mpg and 5th gear results were 10.2 mpg. Both cases were manual throttle trying to be as consistent as possible. I prefer going slower!

David

Re: Most fuel efficient highway speed?

Reply #3 – May 18, 2013, 01:47:19 pm

David, that is good to know. It was 2 or 3 months before I realized it didn't shift into 6th until 61 mph, so I have tried holding near that speed. But I have never gotten 10 mpg.
I will try the 55 spot.

Re: Most fuel efficient highway speed?

Reply #4 – May 18, 2013, 01:52:55 pm

I will give it another try at 55 mph to see if I can get any better than 10 mpg. The main thing I noticed was the torque converter unlocking frequently on every little hill that probably wouldn't be required at 60 mph. My transmission hardly ever downshifts on hills unless they're really steep, it just unlocks the torque converter to raise about 200 rpm, then locks it back up at the top of the hill. I don't know if unlocking / locking the torque converter results in more transmission wear or not. It does raise the temperature of the transmission fluid though.

Re: Most fuel efficient highway speed?

Reply #5 – May 18, 2013, 01:58:38 pm

On a recent 460 mile RT, I kept the speed at about 58 mph and averaged 12 mpg for the trip measuring the tank level before and after within an eighth of an inch. Pretty flat terrain down through the Sac valley and to Monterey, CA. U300, 36 feet long, 102" with a light load.

We get lock-up at 48 mph unless using a bit of throttle.

55 mph in the mountains can easily dip to 7 mpg or even worse in extreme cases. Averaged 6 mpg from Grass Valley to Truckee and back over Donner Pass on I-80.

Pierce

Re: Most fuel efficient highway speed?

Reply #6 – May 18, 2013, 03:52:43 pm

Quote from: RRadio – May 18, 2013, 11:22:13 am

Has anyone noticed a difference in fuel efficiency between driving 55 mph and driving 60 mph on the highway? I realize that driving faster than 60 mph causes a decrease in fuel efficiency.

It is all very dependent upon transmission gear ratios, and rear end ratios. Much of this was determined by engineers at given periods of time when you rig was built. Remember back when all the interstate speed limits were lower? The engineers then used a different final drive gear ratio to keep you mpg high. Now that speed limits are higher, the final drive gear ratios on vehicles being turned out today are different that the days of lower speed limits. So your final drive gear ratios can determine you optimum mpg speed.

Re: Most fuel efficient highway speed?

Reply #7 – May 18, 2013, 03:57:11 pm

You got me again on "torque converter unlocking frequently". What does it feel like when locking/unlocking. I realize that this question is somewhat nebulous however - How is the torque converter different than downshifting/upshifting? Inquiring minds.

Re: Most fuel efficient highway speed?

Reply #8 – May 18, 2013, 06:10:46 pm

Quote from: Carol & Scott – May 18, 2013, 03:57:11 pm

You got me again on "torque converter unlocking frequently". What does it feel like when locking/unlocking. I realize that this question is somewhat nebulous however - How is the torque converter different than downshifting/upshifting? Inquiring minds.

Your torque converter is unlocked in first gear. Locked up in the rest. The early four speeds locked half way through third.

Re: Most fuel efficient highway speed?

Reply #9 – May 18, 2013, 06:35:30 pm

Quote from: Carol & Scott – May 18, 2013, 03:57:11 pm

You got me again on "torque converter unlocking frequently". What does it feel like when locking/unlocking. I realize that this question is somewhat nebulous however - How is the torque converter different than downshifting/upshifting? Inquiring minds.

Feels like a mini shift but the RPM change on the engine is not a much as in a normal shift. To the casual observer, it feels no different than a normal shift. The difference between the lockup and normal is only a couple hundred RPM. The lockup gives you just a little higher ratio than normal and the transmission generates less heat as well in lockup.

Re: Most fuel efficient highway speed?

Reply #10 – May 18, 2013, 06:44:09 pm

Yes, torque converter on Allision 3000 and 4000 series transmissions is UNlocked in 1st gear and low RPM's in 2nd gear. As John said, in 2nd gear you can feel the "mini-shift" as the converter locks up. It is always locked in all higher gears.

That is why the only time an Allison will really heat up (other than using retarder) is in stop and go traffic where the converter is unlocked. If in these conditions, put transmission in Neutral at lights!

Re: Most fuel efficient highway speed?

Reply #11 – May 18, 2013, 10:37:28 pm

Wow - I guess I will have to pay more attention going forward - no pun intended.

Re: Most fuel efficient highway speed?

Reply #12 – May 19, 2013, 12:19:14 am

Posts like these make me appreciate my ISL 400 and that the transmission is geared lower than on the 450hp versions. I am already in 6th somewhere in the 52-55 range. It's been so long since i've been out, I forget the exact speed.

Re: Most fuel efficient highway speed?

Reply #13 – May 19, 2013, 03:36:45 am

Brad,
With the 4.63 gearing due to the smaller engine vs the 3.91 gearing sure would make a larger difference in mph into 6th gear. More important is the rpm the engine is turning.
Either way, it all works out good, due to good engineering by FOT
MHO

Re: Most fuel efficient highway speed?

Reply #14 – May 19, 2013, 08:53:48 am

To paraphrase an old maxim, speed costs horsepower. How fast do you want to go?

I'll give my opinion on the relative horsepower required to overcome air resistance at various speeds. There are other factors such as rolling resistance and engine efficiency, but the wind resistance factor is pretty straight-forward.

The horsepower requirements increase as a cubic factor of speed differential. That's a fact of physics, not an opinion. That means that it takes 8 times the horsepower to double the speed. If a rig requires 25 horsepower to go 40 mph it will require 200 horsepower to go 80 mph. Again, that's the power requirements to overcome wind resistance alone.

Now let's say we have a rig going 50 mph and consuming 50 hp to overcome wing resistance. If we increase our speed from 50 to 60 mph, we're going 1.2 times as fast. Now if we cube that 1.2 (1.2 ^3) we arrive at 1.728. So, our horsepower required to go 60 mph is 1.728 * 50, or 86.4 horsepower.

So now we know that if the wind resistance increases as we increase speed, it should be reduced by the same amount as we decrease speed. If we're using 86.4 horsepower to go 60 mph, and then slow down to 55 mph, we will be going only 0.9167 times as fast. Our horsepower requirements will be 0.9167 ^ 3, or 77% of the horsepower required to run 60 mph. We find that it will require only 66.5 horsepower to run 55 mph.

I seem to remember reading that the trucking industry says that below about 50 mph the rolling resistance has the greatest effect on horsepower requirements, while above 50 mph it is wind resistance and streamlining that are the biggest power thieves.

   Original hp      50
   Original mph      50
   New mph      60
   % speed increase      1.2
   % HP increase      1.728
   New HP requirements      86.4


   Original hp      50
   Original mph      50
   New mph      55
   % speed increase      1.1
   % HP increase      1.331
   New HP requirements      66.55

   Original hp      86.4
   Original mph      60
   New mph      55
   % speed increase      0.916667
   % HP increase      0.770255
   New HP requirements      66.55

Re: Most fuel efficient highway speed?

Reply #15 – May 19, 2013, 10:13:10 am

Interesting indeed. It seems like shape and a factor for gearing and engine power curves/efficiency ought to be a part of it though. Someone with a VMS could pretty easily verify these numbers.

Re: Most fuel efficient highway speed?

Reply #16 – May 19, 2013, 10:50:17 am

Can't even imagine how much fuel our rigs would use without lockup. Coming off the line in first and part way through second gear the acceleration is pretty leisurely. As soon as it makes the "shift" into lockup, it feels like an extra 100 hp to me and. On the way home after we picked up our U300 in N.C., I kept wondering what was wrong in first and second. Sure would be nice to reprogram it to shift part way through first but would probably really increase the driveline loading when it made the shift.

With really steep, two lane road grades, I usually stop at the top and then start down so I am sure what gear, especially which second gear I am in against the gear I want to be in.

Pierce

Re: Most fuel efficient highway speed?

Reply #17 – May 19, 2013, 01:23:47 pm

Keeping this simple, Just drive 10 mph slower than you like and get bored silly while saving a few pennies.

Re: Most fuel efficient highway speed?

Reply #18 – May 19, 2013, 01:26:33 pm

Quote from: Dave M – May 19, 2013, 01:23:47 pm

Keeping this simple, Just drive 10 mph slower than you like and get bored silly while saving a few pennies.

Dave, you are so bad...But, I do 80-85 (would put it to the medal if wife wouldn't look at speed) when heading through west Texas....talk about BORING...esp after the first 4 or 5 times

Re: Most fuel efficient highway speed?

Reply #19 – May 19, 2013, 04:51:32 pm

I will say this. I drive the speed limit. IF you are going slower on a major road you are a hazard. I am not talking a mile of two but 10 is an issue. Also, if you are only going a couple hundred miles, then the speed difference is not a big issue. If you are going cross country, then you have to use speed to your advantage. For instance, i f you drive 12 hours at 55 you will cover 660 or so miles. If you are going 65 mph over the same 12 hours you will cover 780 miles. Now take that 120 miles a day difference and put it over 3000 miles and you get 4.5 vs 3.8 days. So, with stops for fuel crossing the country I can save almost a full day. That means less genset time and that means I save time and that is money for me.

Re: Most fuel efficient highway speed?

Reply #20 – May 19, 2013, 05:41:19 pm

Quote from: Dave M – May 19, 2013, 01:23:47 pm

Keeping this simple, Just drive 10 mph slower than you like and get bored silly while saving a few pennies.

That has been my experience when trying to save fuel. My wife and I are both bored at 60 MPH. (Remember: Unhappy wife = unhappy life.) When traveling at 70 MPH I am more alert, my wife is happier, and so are the other drivers around us on the freeway. I keep track of fuel mileage at every fill-up, and I haven't seen any significant difference in fuel mileage between the two speeds. The Aqua-Hot uses diesel and so it tends to mask relatively small changes in fuel mileage. I am satisfied with roughly 8 MPG overall, and so I simply don't worry about it.

Re: Most fuel efficient highway speed?

Reply #21 – May 19, 2013, 06:38:46 pm

Agreed, doesn't make sense to own a motorhome to save fuel but some folks are just not comfortable driving that fast and they're not all driving Foretravels with good handling and excellent brakes. I'll leave it to someone else to tell em they shouldn't be on the road, some of em swing a mean cane.

Re: Most fuel efficient highway speed?

Reply #22 – May 19, 2013, 07:23:16 pm

Quote from: Jeff Savournin – May 19, 2013, 01:26:33 pm

Dave, you are so bad...But, I do 80-85 (would put it to the medal if wife wouldn't look at speed) when heading through west Texas....talk about BORING...esp after the first 4 or 5 times

Jeff's not telling the whole truth. When I am following him he goes between 50 mph and 80 mph then 60 mph then 80, the 55 then 65 then 80. One day he will use the cruise control, maybe.

Quote from: John S – May 19, 2013, 04:51:32 pm

Now take that 120 miles a day difference and put it over 3000 miles and you get 4.5 vs 3.8 days. So, with stops for fuel crossing the country I can save almost a full day.

You can tell John's not retired. Who drives over 3 hours per day anyhow.

With my Detroit 6V92 it doesn't seem to make much difference whether I'm going 55 mph or 75 mph. I have checked the mileage at 5 mph increments on several trips and I'm guessing that elevation and wind make more difference than speed. I understand the physics but when you include the influences on mpg of the gearing vs. torque curve vs. environment I just drive the speed I desire based on the traffic and road conditions. .

Re: Most fuel efficient highway speed?

Reply #23 – May 19, 2013, 07:29:39 pm

I hear you John. Your destination is the destination. I'm impressed with what you'll do for family.

But our destination is usually the journey. And we rarely drive 6 hours a day. And I rarely get bored driving. I find nature infinitely interesting. And no one gets bored around Kathleen... She's a treat!

As far as speed limit goes I often drive the speed limit on I-5 in CA, which is 55mph when towing. But I will speed up to 60 or so if it gets so congested truckers are unable to pass us. (It's pretty obvious some of the big companies are limiting their trucks to 60 or so, Wal-Mart being one. And the Internet makes it hard for drivers to cheat.) I'm too risk-adverse to cruise faster than that. But I will go up to 70 a bit coasting down hill.

I'm too risk-adverse to drive a motor home at night, too. Only did it once or twice in 10 years.

best, paul

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I will say this. I drive the speed limit. IF you are going slower on a major road you are a hazard. I am not talking a mile of two but 10 is an issue. Also, if you are only going a couple hundred miles, then the speed difference is not a big issue. If you are going cross country, then you have to use speed to your advantage. For instance, i f you drive 12 hours at 55 you will cover 660 or so miles. If you are going 65 mph over the same 12 hours you will cover 780 miles. Now take that 120 miles a day difference and put it over 3000 miles and you get 4.5 vs 3.8 days. So, with stops for fuel crossing the country I can save almost a full day. That means less genset time and that means I save time and that is money for me.

Re: Most fuel efficient highway speed?

Reply #24 – May 19, 2013, 08:21:13 pm

Well, I just drive the speed limit. I also try not to drive at night any longer. I will drive in the early morning dark though.
I will say I like to drive and look at the scenery around me as well. Then again running I 95 there is none.

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