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
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
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
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.
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.
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
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.
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.
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.
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!
Wow - I guess I will have to pay more attention going forward - no pun intended. :)
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.
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
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
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.
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
Keeping this simple, Just drive 10 mph slower than you like and get bored silly while saving a few pennies. ;D
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
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.
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.
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.
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.
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. .
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
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.
...wow, thanks for all those answers, even to stuff I didn't ask ;D I can't drive slower than 50 mph because my transmission doesn't shift into high gear until about 48 mph and I'm sure it's most fuel efficient in high gear. I suspect 55 mph might be more fuel efficient than 60 mph on flat land, but it seems like the torque converter unlocks more often on little hills at 55 than it does at 60, which sorta loses whatever advantage there might be. A lot of fleet vehicles are governed at 60 mph now, so I presume it's the most efficient speed for a lot of those vehicles or the owners wouldn't have done it. I'm running my tires at Foretravel's recommended 90 psi or slightly higher. The previous owner experimented with higher pressures but said he didn't notice any advantage. He told me it rode a lot harder so he reduced the pressure some. When I bought the coach he had the tires at about 95 psi. I jumped to the conclusion that wind resistance was the major fuel efficiency factor and never messed with the rolling resistance. I drive with about 25-50 gallons of fresh water and empty gray and black water tanks. My cargo bays and cabinets aren't fully loaded and I'm carrying a motorcycle instead of towing a car. I suppose any reduction in weight is an improvement in fuel efficiency and hill climbing ability. Foretravel doesn't disappoint in either category. I pass every other coach going up steep hills, but they all pass me again going down the other side. Every other coach owner I've talked to says they're getting significantly less than 10 mpg. Most of them are in new Phaetons with flat noses. I even spoke to a guy with a pickup truck that barely got 10 mpg without towing a trailer the other day. Basically everyone is amazed that I get 10 mpg. Foretravel obviously knew what they were doing and optimized everything.
When I was in a truck repair shop last month a driver was there trying to get his engine reprogrammed to allow 70mph since the speed limit on US 59 was just raised to that point. It took a lot of discussion between the service manager and a decision maker at the trucking firm. The shop was finally allowed to re-program to 65mph on cruise but only 60 off cruise. Do you think these companies are taking fuel mileage seriously? It's something to see trucks crawling across west Texas at 60mph when the speed limit is 80. Didn't used to be that way.
Sometimes it is for accident control and insurance too. They get a better rate if they are governed.
I'm never bored with any of our trips over the past several years. I believe that if I needed to get somewhere faster than the 57 mph sweet-spot I'd simply take a plane and get the "awful" drive over with. The whole trip is the trip to us and we enjoy all the things we see and enjoy the things we stop to se between point A and B. I retired and learned that the yellow means slow down rather than speed up. I learned that what was passing me by in the front and side windows was the actual reason for this lifestyle. I learned that pennies saved were, in fact, pennies earned. We only go above the sweet spot when going down hill and even then I pull on the retarded and wonder what in the world we're going so fast for.
Howdy Larry,
Well said, sir!!!
Dave A
They also get lower insurance rates.
This confirms my point. Go faster. Increase risk. Going slower is not just related to saving fuel/$$$.
best, paul
My 6th gear locks in at 62mph. Then in cruise control I can bring it down to 60mph and stay in 6th in rolling hills (with 'mode' on).
But my VMS 240 CL surprised me when it showed higher mpg at 55mph in 5th gear.
As an aside, I discovered my VMS 240 CL to be an important safety device. All the engine stats are right there in front of my eyes, changing and changing from road circumstance to another. My VMS changing and the scenery changing keep me awake...
But then, being so engrossed in RPM, torque vs speed, recent MPG, seeing if I can squeeze 0.1 mpg more out, etc, etc, is not my fault. I'm an engineer. They beat it into us ;o)
Pity Kathleen. No one ever warned her about engineers.
But then, no one ever warned me about redheads either (running and ducking...)
best, paul
If you have a trucking company with 1000 drivers and you decrease the MPG of each truck by only 1MPG you can save millions of dollars a year, yes it is only pennies for each day for each truck but my Mom always said, "Watch the pennies, the dollars will take care of themselves.
1000 Drivers
20,000 miles per year
Total miles 20,000,000
@6MPG 3,333,333 gallons of fuel
@5MPG 4,000,000 gallons of fuel
@ $3.00 per gallon @6MPG 9,999,999 @5MPG 12,000,000
Decrease of 1MPG per driver saves a little over $2,000,000 in fuel alone
When I was a company driver, the company rewarded folks for low IDLE times just to save PENNIES. Their trucks were also governed at 60MPH. Schneider at that time was governed at 55MPH. You can adjust schedules to compensate for longer driving times, but you cannot recover the fuel that is burnt needlessly.
I know some folks say they "get sleepy" when they drive 55 or 60 MPH. If that's the case stop and smell the roses, let the DW have a turn at the wheel. That's what we do, we also avoid interstates whenever we can. Got into the fulltiming lifestyle to see this country, not watch go by through my windshield.
Roland
A dollar a day each truck, for a fleet of 15,000 tractors sure becomes an interesting number each day, Think large fleets such as Schneider, UPS, FedX etc.
Why the bean counters & insurance become a force to deal with.
Thats funny!!! I quit driving over the road as a independent in 1983 when I took a regional delivery job for a union outfit hauling bulk petroleum products. One of they things they told me was "this company runs legal"! They told me "we don't speed"!! Also told me "we don't lie on our log books" !!! And then they also said "you don't need to hurry"!!! I thought they were nuts, but come to find out, that was how they operated. I had a heck of a time slowing down
Interesting, that no one mentioned the optimum operating RPM (found in your engine manual) for a given engine. This is the point where max torque and horse power meet. Not all engine engines do well at lower RPM's (you 2 stroke guys that know well) My ISC is like 1650-1800 RPM fuel consumption aside the engine runs cooler and better oil pressure. By useing the manual shift for the transmission when on secondary roads, city traffic, it helps minimize low RPM's and excessive shifting.
The reason I bought my Foretravel in the first place is the comfort factor ! All the little things that enter into that include the speed that I drive . I do tune into the MPG , but that is not my driving force .I see that RPM is what shows the best or worst MPG to a point then MPH also enters as pushing or pulling air has it's effect .Back to comfort , yes that is my thing . The road , traffic and general road conditions are what is my comfort thing . I have no problem with 55 MPH as well as 75 or so MPH .Not a bean counter nor a complete dumb butt , so MPG is important ,but driving comfort and feeling good going down the road is why I have a coach in the first place . I know this is a different strokes for different folks thing . Brad Metzger
Agree with Brad...This is a very individual pursuit. MPG has always been a "thing" with me...must be the engineer thing coming out...it's just ingrained in my mentality to seek the most efficiency from anything. Engines lend themselves easily to this king of thought. It really is not that critical a statistic insofar as motorcoach driving goes. Let's face it, we just don't drive that many miles to make a world of difference.
e.g. I typically drive the coach 10,000 miles per year; and some years only 5,000 miles. The variance in mpg I've observed over 10 years is between 7.5 mpg (Power setting on Allison. towing and in hilly high altitudes) and 9.0 mpg (using MODE on Allison). At $4.00 per gals. this is a total cost difference of $915 for the entire year/season if driving 10,000 miles, and only if I were to change driving habits for an entire year. Granted, to some the $900 difference is important. And if you drive 40,000 miles per year, then it does begin to be a few more bucks than even I am willing to "burn"...no pun intended... :o
I look at mpg more as a "am I driving right" and "is everything running as it should" more than "Can I save a few buck this leg of the trip". Otherwise my RV would be a TDI Jetta or something similar.
I don't know about that Jay??? As I remember, the guys who were getting the really high mpg with their TDIs were getting lots of oily crap coming through their EGRs and plugging up their intakes super bad. But if you ran the heck out of the 1.9 ALH diesel, it will not plug up.