Engine/Transmission (1998.5 - 2002) Most economical RPM for a 24V?

[VicR]

Another thread on MPG got me to thinking - what is the most economical RPM for a 24 valve? It always seems that I do best between 1900 & 2000 RPM, but is there a way to figure out exactly the sweet spot? I thought the power curve graph would answer my question, but if it is I'm missing it.



Thanks,



-Vic

 

[SnoKing]

I think it would depend on the gearing. With 4:10's the 19-2000 is close. With 3. 54's you are starting to push alot of air out of the way at 19-2000 RPMs. SNOKING

 

[Cummins Pilot]

To get really picky about it get a boost & pyro gauge. Run a in a gear that spins the engine the slowest while keeping around 6-700egt & low boost 5-6psi. This is where they seem to run the best. As the boost and temp climb your fuel mileage will go down. To high of a gear and the boost will be low but the egts will climb putting a lot of strain on gear train and using extra fuel. I've found that 19-2000 rpms is pretty good for fuel mileage on mine.

 

[AFoulk]

mine seems to do the best at around 1500-1800rpms, which equates to 50-55mph with my 6-spd, 3. 55's, and 265/75-16's.

 

[SNiebaum]

1850 RPM or so seems to be my cruising sweet spot... just shy of 70mph with my setup... 3. 55's, 6 speed, BFG 285/75/16's...

in town, I drive by my boost and EGT gauges and of course the speedometer...

 

[johncameron]

1850 RPM or so seems to be my cruising sweet spot... just shy of 70mph with my setup... 3. 55's, 6 speed, BFG 285/75/16's...



in town, I drive by my boost and EGT gauges and of course the speedometer...

Ditto



Keep ti below 70 and that seems to be the trick, wind KILLS MPG.

 

[johncameron]

I'm using Micheline XPS traction tires, they are extremely hard and seem to help MPG a hair. It is hard to be sure because I added the SMARTY in the same time. I can cruise at 70MPH between 550-650F on SW1. I'm sure my 2WD helps also.



John

 

[Bob Cochran]

Has anyone weighed the XPS compared to other tires? I have not but can say when I removed the Micheline LTX and put on the XPS. The XPS's weighed more so would that not lower fuel mileage. Just a thought that came spilling out.

 

[johncameron]

Has anyone weighed the XPS compared to other tires? I have not but can say when I removed the Micheline LTX and put on the XPS. The XPS's weighed more so would that not lower fuel mileage. Just a thought that came spilling out.

I never weighed them but they are quite a bit heavier.



I would say a harder-skinnier tire like the XPS should have less rolling resistance which should = better MPG on the highway. In town stop+go it would take more power to get it moving for sure.



JC

 

[SnoKing]

They do have a very low rolling resistance. Chris

 

[LSchulz]

mine seems to do the best at around 1500-1800rpms, which equates to 50-55mph with my 6-spd, 3. 55's, and 265/75-16's.

Same here, with an identical drivetrain.



Best I've been able to get was 21 mpg, which I'm hoping I can still improve. By contrast, my record with my last truck was 22. 3 mpg ('02 F350 longbox CC, 4x4, 6 spd, 3. 73's, 265/75-16's, on winter fuel). That said, I've tended to be a little less patient in getting from point A to point B these days.



Either way, it's better than the bus, which we've been running as hard as we can on the highway for the last 10 weeks. The DT466 gets about 6. 5mpg at 2500 rpms.

 

[Froadin]

6-spd, 3. 55's, and 265/70-17's. (stock Michelins from 2003 truck on my 2001).
Best mileage to date, 23. 899 mpg.
Best rpm/mph is 1800 ish, and 65 to 70. Ideal speed seems to be 65 to 67, but that is splitting hairs really.
I can make a trip through the Siskiyous down to Redding doing 80+ for most of it and still get 20. 5 mpg.
I have these tires at 70 to 80 psi though (was testing for mpg purposes, but leave it there now).
I average about 21. 5 mpg now.
With the winter fuel and studded tires, I was around 18 mpg
2wd is a nice thing for mpg, and a short bed single rear wheel helps.
The 05 is between 16 and 19 mpg now, highly variable with it though.

 

[2broke2smoke]

1800 for mine ... 65 mph ...

 

[isb360]

I thought I remember seeing some charts a couple of years ago, perhaps in printed TDR that graphed power, torque and BSFC (brake specific fuel consumption), or how much fuel is being consumed per horsepower produced at different rpm's. Seeing that curve would help clue you in on optimum operating rpm.



Tim

 

[Froadin]

It was because of that exact graph that I started driving at speeds/rpms within the range to find the optimal mileage. It's on here somewhere.
1800 to 2200 was the 'big window' where we are efficient. And I think it was 18xx rpm for the best?
But we start pushing a LOT of air when we go that much faster. And wind resistance really adds up.

 

[VRidgley]

I stumbled on the issue of optimum RPM in this thread while researching another question.



Basically, the flat torque curve in the 24v 5. 9 HPCR from 1500 to 2500 gives you a large window of possible maximum fuel economy. What will affect it most is ambient and/or forcibly pushed wind (a function of speed), and the particular gearing you are running at a given speed.



I am running a Gear Vendor overdrive in a 2006 6-speed, and have constructed many spreadsheet graphs reflecting the impact of torque on available speed ranges in different gears. Look at the pdf file, which shows in one graph the speed ranges available in each gear, regular and overdrive, and in another graph the likely hp output as a function of torque. (The other two pages are raw numbers used to create the spreadsheet graphs).



At present, I am making 20-21 mpg at 75 mph, 1900 rpm, on I-80 back and forth across Nevada. Realize, however, that I was forced into buying the Gear Vendor unit because Dodge did not offer a 3. 55 axle in 2006. which is what I normally would use with a 6-speed manual, so I am stuck with the factory 3. 74. Those of you who have an automatic have a transmission with intrinsically higher gearing than the G56, so you normally get higher mileage by default, without having to resort to an add-on overdrive.

 

[Froadin]

The only problem with that is that 'flat torque curve' is only available like that at WOT.
The REAL test is what it would look like if faced with the proper wind resistance while 'cruising'. Which would tell you how much more fuel you use.

IF someone has a monitor that reads all of these, these engines do tell us engine load, etc. Now that would be a great thing to view while driving!!!

 

[VicR]

Interesting info Vic - I printed it out and will study it tonight.



One interesting observation from my trip last week. I spent last week at sea level (highway 1 in No. Cal) and the sweet spot 'seemed' about 250-350 RPM less then it is up here in the Sierra's. I wonder if altitude is also factor in MPG as it is in power?



-Vic

 

[AKaiser]

Best fuel consumption will occur at the lowest HP that will get the job done without overheating/lugging the engine. Generally, if you're not towing, keep it down at the lowest point on the max torque curve. Most of these trucks, it's 1600. Much below that, you're going to experience power losses due to the piston being too slow. The higher you go, power losses due to internal friction. May be irrelevant to 2Gers, but that's why the 68RFE has 2 ODs and 3. 73s and 4. 10s as an option. Unless you're pulling heavy, you're going to wish you had the 3. 73s for the fuel consumption.



Check this out. Clarke Echols