3rd Gen Engine and Drivetrain -> 2003-2007 5.9 liter Engine and drivetrain discussion only. PLEASE, NO HIGH PERFORMANCE DISCUSSION!

fuel economy, engine rpm, axle ratio...and sweet spot

Old 01-26-2003, 06:33 AM
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fuel economy, engine rpm, axle ratio...and sweet spot

When I was at the dealer "negotiating the deal"...I asked to see an engine HP and Torque graph. It seems the ctdHO has a consistant and constant 550ftLB of torque between 1400 and 2600 rpm. The Torque graph started around 250ftLB and gained to 550ftLB at 1400rpm, held there until 2600rpm then started to drop again. (For those who are familiar it looked like a parabola cut in half and you remove the quadrant portion.)

So this being said and since I am a motorhead I started thinking that for any fuel the engine peak torque rpm is also the point of peak fuel effiency. Given the 03 cummins has this broad flat torque peak has anyone noticed any substainial fuel savings or losses based on engine RPM in top gear?

So then I studied the horsepower curve. The peak horsepower rating of 305hp was closer to 2800rpm. The hp and torque graphs crossed around 2200rpm. I am thinking that since Dodge presents the graph this way they have thought about it I am now thinking about it. I think the ctdHO sweet spot is between 1800 and 2600rpm as the 6spd transmission rpm drop during shifts is approximately 800 rpm and was wondering what the experiences on the board is?

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Old 01-26-2003, 07:03 AM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

Do you have a copy of that chart....that you could scan and post here...it would be interesting to view it
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Old 01-26-2003, 09:04 AM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

[quote author=Hummin Cummins link=board=20;threadid=10122;start=0#96672 date=1043584412]
The hp and torque graphs crossed around 2200rpm.

[/quote]

Don't let Dodge's graph confuse you on this...all horsepower/torque curves cross at 5252.1 rpm, no exceptions. Has to do with the way horsepower (a calculated number) is derived from torque (a measured number). Dodge is throwing you a curve in that there will be an axis on the left hand side showing values for horsepower and another axis on the right showing values for torque. What you're really seeing is two graphs superimposed on one another which have different "y" values. Where the curves actually cross each other is meaningless unless the "y" values are the same.

Also, these curves are for wide open throttle, a condition which doesn't really apply when cruising. In general, though, to get better mileage, slow down. It takes eight times as much horsepower (and fuel burned) to overcome aerodynamic drag (the biggest component of fuel consumption) at 80 mph than it does at 40.
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Old 01-26-2003, 11:05 AM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

You're correct on the bhp/torque curve. For any engine, the &quot;sweet-spot&quot; is where those curves cross. These curves are derived from an engine brake dynometer, and DO vary from engine to engine. I have worked with engine dynometers from as far back as 1952 when I worked for GM Diesel in the experimental lab. In facdt, in late February I will be putting a Ford Focus engine on an engine dyno in Southern Cal to tweek up the timing and injectors before putting it into a Stealth midget chassis up in the Bay Area.<br><br>As I stated in another link, best fuel efficiency is derived between 65% and 75% of red-line RPM. In this case (with a red-line) of 3200, these RPM's are between 2080 and 2400, So 2200 hits it right on the head.<br><br>Now, for over-the-road efficiency you have to figure in such factors as truck wieght, trailer weight, frontal area, streamliing, temperature and wind velocity. With my last pick-up, I found that when towing my 30', 8500# race trailer, that the addition of a spoiler on the rear of my camper shell increased my fuel mileage an average of 15%. The new truck appears to have better streamlining on the snout, hood and windshield, so I would assume the fuel mileage will be even a little better.<br><br>Many people don't take the wind velocity into conideration and it can also have a huge effect on fuel mileage. Just think, if you're heading east at 65 mph, and you have an east wind at 30 mph, it's almost the same as driving at 95 mph (a little less because of the rolling friction), but if you're headed west with the same wind it is like driving at 35 mph (a little more because of the rolling friction which will always be a function of your indicated speed(.<br><br>Hope I confused you --- ;D
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Old 01-26-2003, 11:22 AM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

Here is more food for thought.... calcuating the wind drag co-efficiency.... While pushing a wall of wind in a cabover @ 55 miles per hour...the wall of air mass is almost 90 degrees. To find the sweet spot on fuel economy, you would have to listen to the engine and try to hear the differance between working and cruising. With more aerodynamically designed trucks you really have to feel when you are pushing (working) it. It takes a little time and experimentation.
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Old 01-26-2003, 12:27 PM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

Getting back to the original question, I think most owners are finding that fuel economy drops off fairly rapidly past 2000 rpm.
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Old 01-26-2003, 09:47 PM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

[quote author=balvert link=board=20;threadid=10122;start=0#96750 date=1043605674]<br>Getting back to the original question, I think most owners are finding that fuel economy drops off fairly rapidly past 2000 rpm.<br>[/quote]<br><br>Pretty close in that statement. Playing with the computer at various speeds for 20 mile segments on the Interstate, I was at 20.7 at 60 (2000 rpms) and 19.9 at 70 (2200 rpm). That is with a 4.10 axle and six shooter. I felt very comfortable in those ranges. The 4.10 really comes into play when towing the fiver. If you are towing much, go 4.10. I did notice a significant drop at 80, over 2 mpg from 60. <br>Bob
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Old 01-26-2003, 10:03 PM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

Bob,<br><br>Why not get the 3.73's and when towing, just use 5th gear or is that too much of a drop?
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Old 01-27-2003, 10:06 AM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

I have not been scientific about it but since I have to pay the fuel bill I have noticed what my truck likes.<br> Towing my fiver it likes it around 70 mph. I have not really tried it faster but slowing down a little seems to have no improvement in fuel mileage. I am sure if I slowed at some point it would improve.<br> I am assuming it has something to do with the turbo charger and the way it effects power with the Cummins. Can anyone explain to me why my truck would do just as well at 70 mph on fuel mpg say versus 65 or little less pulling a 12&quot; tall fifth wheel? <br> I know everything on paper might say otherwise but my truck's mileage say's different.<br> Randy
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Old 01-27-2003, 10:21 AM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

Pappyman, that chart is posted on the cummins web site for Ram trucks. <br>Hummin - My '99 does get better milage at 1900 to 2000 RPM, like most posts I have seen here, and I will be interested in seeing what I get out of my new one when I get it. I went with the 3.73 because I only pull the 5ver six or seven times a year and don't really need the 4.10's for that. If I were hauling very frequently, I would have chosen the 4.10 diff's. After driving for a while, you will find the right spot for you and your kind of driving. Enjoy.
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Old 01-27-2003, 07:40 PM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

[quote author=cp link=board=20;threadid=10122;start=0#96879 date=1043640226]<br>Bob,<br><br>Why not get the 3.73's and when towing, just use 5th gear or is that too much of a drop?<br>[/quote]<br><br>My biggest problem was a 16000 pound fifth wheel. You need the 4.10 to get the tow rating - - 3.73 does not cut it to stay legal. And, I don't run much over 70 solo in the truck, so I am in a good range on the rpms. The biggest reason though is the towing. 60 at 2000 rpms with that monster on behind - - perfect. Feels really good at 60 to 65 and keeps the rpm where I am getting good mileage. Then, I have a little tighter spread in my gearing when the road heads for the sky. :P
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Old 01-27-2003, 08:31 PM
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Re:fuel economy, engine rpm, axle ratio...and sweet spot

GrandpasRam,<br> We are not far apart I noticed you live in Waxahachie, I am in Midlothian.
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