Speed vs. "sweet spot"

[allan5oh]

I did a test last trip which is as follows:

Ran 55, 60, 65 mph in both 12th and 13th gear, all for at least 10 miles on completely flat terrain (I-29 in North Dakota) with steady conditions. I reset the computer MPG every time, and established an average. I even re-ran some of the tests, all were either repeated dead on or within 1/10th of a MPG.

Those of you that say computers aren't accurate I say this: They aren't accurate, but they will *always* be out the same amount. If your computer reads 6.0 mpg, but you're actually getting 6.5, and you slow down now the computer reads 7.0 mpg, you damn well know that it has improved.

The results:

1) Coming out of the sweet spot cost at least 1/10th of a MPG and at most 3/10th of a MPG. At 55 mph the best MPG was achieved below the "sweet spot" in 13th gear, although in 12th I was right in the "sweet spot".

2) An increase of 5 mph cost me 8/10th of a MPG. Far more difference then running in or out of the "sweet spot".

I have come to the following conclusions:

a) "I need to run 65 mph to be in my sweet spot" is pure hogwash! Decreasing the HP requirement on the engine is far more important then running right smack dab in the sweet spot.

b) "sweet spot" itself is not what everyone thinks it is. The best MPG will be achieved well below this RPM. Even the Cummins white paper states it... the sweet spot is the best compromise between MPG and "driver satisfaction" (aka pulling power).

c) Speed is and will always be the largest factor in MPG.

Here are the hard numbers:

55 mph:

7.8 MPG in 12th gear at 1508 RPMS
7.9 MPG in 13th gear at 1279 RPMS

60 mph:

6.9 MPG in 12th gear at 1645 RPMS
7.2 MPG in 13th gear at 1396 RPMS
65 mph:
6.3 MPG in 12th gear at 1782 RPMS
6.4 MPG in 13th gear at 1512 RPMS

I'm pretty sure the published "sweet spot" of the engine is 1400-1500 rpms.  Obviously that's BS.

[Les2]

I got one that will make you think!

This was all running the same loads, same area for about 3 or 4 yrs.

trans...RTLO14613
1800rpm, 72mph...6.8mpg
1500rpm, 62mph...6.8mpg

Changed trans to lower rpm at higher speeds
RTLO16913A
1500rpm, 72mph...6.8mpg...OD
1500rpm, 62mph...6.8mpg...Direct

I have gotten over 7mpg's running flat areas or with light loads. I've also gotten worse in bad conditions.

I was hoping with the transmission change I'd stay above 7, didn't seem to matter, neither did the speed!

[JR OTR]

Aerodynamic resistance goes up as the square of the speed, though it starts as such a small number you barely feel it (think how much you feel with your hand out the window at 20 MPH vs 60 MPH).

Below 55 MPH things like gearing and load weight can matter more than speed itself, but over about 55 MPH (particularly in conventional tractors) nothing comes close to mattering more than air resistance.

Slow down, save money.

Jim

[firebird_1252]

figure this one out.

the company i drive for last year just turned all the trucks back to 65.

the specs of my truck are:

06 columbia
single axle
10 speed
3.58 rear
spoiler not full fairings
515 s60 (no idea why).

anyway, i run real light and alot of stop and go.

when the truck was still turned up to 515 hp it ran 74mph, i'd normally get 6.8-7mpg

once ryder took over the leasing company was the same time my boss knocked the trucks down to 65. they also castrated (and man does it suck!) the hp down to 455 and 65 mph. last time i checked i was getting 7.25mpg.

what i noticed is that your rpm range when you shift has ALOT to do with mpg. i was tired and cranky yesterday so i was taching the hell out of it and i'm down i'd say about 50miles from where i normaly am. i usualy shift around 1400. yesterday i was shifting around 1800-2000.

[heavyhaulerss]

figure this one out.

the company i drive for last year just turned all the trucks back to 65.

the specs of my truck are:

06 columbia
single axle
10 speed
3.58 rear
spoiler not full fairings
515 s60 (no idea why).

anyway, i run real light and alot of stop and go.

when the truck was still turned up to 515 hp it ran 74mph, i'd normally get 6.8-7mpg

once ryder took over the leasing company was the same time my boss knocked the trucks down to 65. they also castrated (and man does it suck!) the hp down to 455 and 65 mph. last time i checked i was getting 7.25mpg.

what i noticed is that your rpm range when you shift has ALOT to do with mpg. i was tired and cranky yesterday so i was taching the hell out of it and i'm down i'd say about 50miles from where i normaly am. i usualy shift around 1400. yesterday i was shifting around 1800-2000.

if you shift at 1400, I would think you would be lugging the engine? up shifting at 1400 would bring you down to 1100-1200 range...

[firebird_1252]

if you shift at 1400, I would think you would be lugging the engine? up shifting at 1400 would bring you down to 1100-1200 range...

its really not too bad. granted its alot easyer over 1400 but, its not bad at all. yes when you do shift its like 1100 or 1200. again like i said i run us mail so i'm light 90% of the time. my truck is a 14l s60 and it actually pulls out real well. i was using a 12.7l egr s60 a while ago and it ran alot better shifting at 1400 then lets say 1700.

[rgordon212]

Unfortunatley, if you talk to any organization that has spent millions upon millions of dollars trying to figure out the best way to get the best MPG, they will all tell you the same thing...... driver's are 80% of an MPG.

This means that even tho the driver may be in a truck set to cruise at 65mph to get the best mpg..... the way and manner he gets it up to 65mph will make up to 80% of the total. Make sense?

Now I dont pull loads over 30K lbs , and I have a 05 INTL 9400i with prosleeper, no farrings, 3.23 rear gears, 0.70 final ratio and a C15 and I average 6.5-6.0 all day long. I keep my speeds between 60-70 depending on the load.
I recently have been pulling loads weighing 6000lbs total and have avg of 6.5mpg at 68-70mph from Dallas to Miami.

Thing is, with a light load, my fsc is paying for ALL of my fuel so it really makes no difference to me.

[allan5oh]

This means that even tho the driver may be in a truck set to cruise at 65mph to get the best mpg..... the way and manner he gets it up to 65mph will make up to 80% of the total. Make sense?

It's the exact opposite. You spend the majority of your time at cruise, so the speed the driver chooses will have the most effect on MPG. It takes less than a gallon to get up to full speed even with a full load. Accelerating slower will not have anywhere near the effect that slowing down your cruise speed will.

[Barney the Cabover]

8.127330527 mpg year to date... give or take. Gallons pumped vs. gps miles. Accurate as you can get



'02 Intl 9200 with Tri-Pac apu. ISX Cummins @ 400. 10-speed direct with 2.64 rears. Just over 1 million on the engine with no overhaul. 57 mph and never one over. I never shift over 1400 until the last shift into 10th. And that one is at about 1425. I cruise all day @ 1289 rpms.

My payloads go from 8K to 45K in a dry van. The average payload per mile (with deadhead figured in 0lbs) is 30,822 lbs. Mostly midwest to southeast, some northeast.

Laugh as you pass me if you want... I'm headed to the bank. LOL

[Kranky]

I cruise all day @ 1289 rpms.

Have you got a digital tach?

.

[poster]

I'm pretty sure the published "sweet spot" of the engine is 1400-1500 rpms.  Obviously that's BS.

Is there any argument that engine components may fail sooner rather than later if the engine isn't operated in the published sweet spot?

[allan5oh]

Unless the RPMS are really low I doubt it. Electronic engines are pretty good. I think if you're light 100 rpms below the "sweet spot" is fine.

[Kranky]

Next thing I'm expecting to see on here is someone saying that they cruise all day @900 RPM.

.

[Barney the Cabover]

Have you got a digital tach?

.

No... it's pretty simple math... 57 mph on my GPS. The equation is
Tires=514 rpm (rotations per mile) x 2.64 (rear gears) = 1356.96 x 1.00 (tranny final ratio) = 1356.96 rpm @ 60 mph. 57 mph is 95% of 60 mph. 1356.96 x .95 = 1289.112 rpm.

My tach actually shows about 1375..

[solo379]

Accelerating slower will not have anywhere near the effect that slowing down your cruise speed will.

It depends...Some times, actually quite often, i have to "accelerate" a hundred times a day.....

[solo379]

Thing is, with a light load, my fsc is paying for ALL of my fuel so it really makes no difference to me.

Wow! It makes no difference to you, if that money go out of staks, or into your bank account?

[coastie]

With my Truck I tried it, and it did not matter what speed I went, I got the same bad fuel milage, 5.56. 55mph or 75mph. So I said why be a road hazard and go with the flow.

[Bulldog]

I simply found the very first post interesting b/c the poster did'nt take into account the other variables that others have already mentioned. Not every truck has a 13 speed or higher trans, everyones rears are different, and loads carried varies from 1 truck to the next. I know with my truck, a mid-roof volvo, w/ a straight 10 eaton, volvo d-13, 3.73 rears, and 48,880 load, my truck gets about 5.5 on level ground loaded. I never carry less that 47,000, and dead head back to the terminal when finished. My truck is goverened at 65. I average 5.8 over the length of any given run. On the other hand, another truck in the fleet with all the same specs, but having the ISX 600 gets better fuel economy than I do, and the empty combination weighs 1000lbs more than mine? I'm no math wizard, but I say to each his own, and for the record, with my setup, 65mph is at about 1550 rpms and still within the sweet spot zone. slower does improve fuel economy, but there is a point where one has to put good sense to use over fuel economy. If you get in an accident trying to save fuel going slow, than all the money you saved will just go to help pay your lawyers fees.

[eplurubus]

it did not matter what speed I went, I got the same bad fuel milage...So I said why be a road hazard and go with the flow.

Same here. I get 4.5 whether I do 55, 65 or 75. So, guess which one I'm gonna go with (hint: it's not 55 or 65).

[eplurubus]

the speed the driver chooses will have the most effect on MPG.

It has the most effect? How much more effect does it have than weights upwards of one-hundred thousand pounds? How much more effect does it have than 550hp+ fuel guzzlers? How much more effect does it have than hefty crosswinds or headwinds? I could keep going with the questions, but I'll stop there and agree that speed has 'some' effect, but not 'most' effect.