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Is there an 'optimal' RPM for mileage?
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Author:  bebey [ Tue Mar 01, 2011 6:59 pm ]
Post subject:  Is there an 'optimal' RPM for mileage?
Info: 2006, lifted, 245/75/16 MTRs, stock gears.

Is there an 'optimal' RPM for fuel mileage in the CRD? Am I better off going 55 at 1800 RPM and the F37 transmission not shifting into it's final gear, or am I better off going up to 64MPM when it shifts into highest gear and again, is at 1800RPM.

I am planning on getting both the engine and transmission tune from GDE shortly, but in the meantime, I'm hoping to better understand the level of significance of each for RPM, gearing, wind resistance, tire weight, lift, etc. on mileage, and which ones have the greatest impact.

Thanks!
Author:  BlackLibertyCRD [ Tue Mar 01, 2011 7:52 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
2,000 RPM seems to be the sweet spot.
Author:  bhc04 [ Tue Mar 01, 2011 7:56 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
IMHO, you're better off going any speed with minimum r.p.m.'s. That's "usually" the rule of thumb. On my 06 CRD I've found driving on fairly flat highway against no head wind, for optimal milage about 67-68 mph, rpm's around 2k. I don't have any lift or extra equipment on the exterior to create any drag other than stock. I run approx. 38 lbs. in 235x70x16 Khumo K78's which is rated an all terrain tire. Drove last year from my home in Bullhead City AZ. to Ferndale WA. (way upper state close to Canadian border) for Thanks Giving week and the whole trip averaged just a tick above 27 mpg. Drove Highway 5 most the way, some rain, snow and dry weather.
Author:  ATXKJ [ Tue Mar 01, 2011 8:42 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
http://www.vmmotori.it/uploads/doc/29.pdf

scroll down to page 13 and zoom in on the line labeled BSFC

That's the amount of fuel used for a given power output so ~ 2000 rpm is the min.

now you'll still get lower fuel usage below that - however the power dropoff is more significant

so the most general principle is the lowest speed in the highest gear you can hold.

but 2000 gives the best balance
Author:  Pote [ Wed Mar 02, 2011 3:59 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
I think theres a test you can do. Drive up to speed just where it shifts over to your top gear, then back off to where it just stays in 5th....for me its exactly at 61 mph. I think I'm cruising right at 1900-2000 rpms...i'm a gasser though. Give it a try. Basically in 5th at minimum rpms, that's where you get maximum mpgs at highway speed.
Author:  warp2diesel [ Wed Mar 02, 2011 8:20 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
ATXKJ wrote:
http://www.vmmotori.it/uploads/doc/29.pdf

scroll down to page 13 and zoom in on the line labeled BSFC

That's the amount of fuel used for a given power output so ~ 2000 rpm is the min.

now you'll still get lower fuel usage below that - however the power dropoff is more significant

so the most general principle is the lowest speed in the highest gear you can hold.

but 2000 gives the best balance


X2. My non F37 TCM shifts much lower with the RPMs at 1550 at 55MPH. Once I got 34MPG when I drove between Evansville, IN and Aurora, IL driving on state roads in IN & IL. Even with my InMotion Stage II tune that is. Still in the testing stage, don't have the time to compile data and post yet. PM if you want info.
Author:  LibertyCRD [ Wed Mar 02, 2011 9:26 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
In all the testing I have done over the years, 2,000 RPM is NOT the sweet spot for the CRD.

All the data I've collected shows that at interstate speed (70 MPH @ 2,000 RPM) I am getting the same MPGs as I do in mixed city driving. In other words, at that point I've gotten past the point of diminishing returns.

For the CRD...45-55 MPH seems to be the true sweet spot. RPMs will be well below 2,000 and the miles per gallon will be at it's peak.

The diesel does NOT like high speed driving. Parkways, freeways, interstate, etc...not good for mileage. I'm shocked that some people are reporting that high speeds give them the best MPGs.
Author:  Roostre [ Wed Mar 02, 2011 11:05 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
LibertyCRD wrote:
In all the testing I have done over the years, 2,000 RPM is NOT the sweet spot for the CRD.

All the data I've collected shows that at interstate speed (70 MPH @ 2,000 RPM) I am getting the same MPGs as I do in mixed city driving. In other words, at that point I've gotten past the point of diminishing returns.

For the CRD...45-55 MPH seems to be the true sweet spot. RPMs will be well below 2,000 and the miles per gallon will be at it's peak.

The diesel does NOT like high speed driving. Parkways, freeways, interstate, etc...not good for mileage. I'm shocked that some people are reporting that high speeds give them the best MPGs.


My experience is similiar to yours, but VM says otherwise...hmmmm. :?: :?:
Image

I have the GDE TCM and it shifts early and keeps rpm low. My mpg is definitley 2-3 mpg higher than the stock tune. I really dunno what to make of it. I wonder if someone has used a scanguage to actually measure fuel flow in real world conditions- We all know the EVIC is a liar. :?:
Author:  Rixram [ Wed Mar 02, 2011 11:40 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
ATXKJ wrote:
http://www.vmmotori.it/uploads/doc/29.pdf

scroll down to page 13 and zoom in on the line labeled BSFC

That's the amount of fuel used for a given power output so ~ 2000 rpm is the min.

now you'll still get lower fuel usage below that - however the power dropoff is more significant

so the most general principle is the lowest speed in the highest gear you can hold.

but 2000 gives the best balance


I have the GDE ECO Tune and the GDE TCM. I can hold 5th gear lockup down to 52 mph, but I find 55 mph to be a better solution (the buffer gives the cruise control a chance to recover speed lost on hills before a down-shift occurs)

It seems to run about 1550 rpms at 55-56 mph. Not sure on the mileage per gallon, as I've not taken a roadtrip since I put the TCM in...

But last summer, I drove from Rhode Island (home) to northern New York. I had an inflatable boat dingy rolled up on the roof, so aerodynamics were shot to hell, but I averaged 27.6 mpg on that run at 60 mph.
Author:  dieseldoesit [ Wed Mar 02, 2011 11:46 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
Roostre wrote:
My experience is similiar to yours, but VM says otherwise...hmmmm. :?: :?:
I don't think that is what it says. BSFC looks to be a calculation of fuel use per energy output. A lower number means that the unit is in its most efficient state with regards to power output. This is not the same as MPG and the efficiency we think of there. Also, I think the curve for BSFC is for max output (full throttle), which does not occur when traveling at a steady-state speed unless you are going up a large hill with a large load. I think we all know that those conditions are not the best from a MPG perspective. I would imagine that the BSFC curve would get much worse (higher numbers) for throttle settings around 1/2 and a 1/4 of full output.

To determine the most efficient operating region for MPG, we would need to know the actual fuel use (not per power output) for different RPM's and for different throttle settings.

I remember GDE stating that it was more efficient to operate at a higher (like 2k) rpm and less throttle opening then a lower rpm and more throttle.
Author:  LibertyCRD [ Wed Mar 02, 2011 11:59 am ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
All I know is, there is no question that I burn more diesel fuel at 70 MPH than I do on a 55 MPH highway. My mileage really goes up when my cruising speed is kept down.

Also, the TCM tune from GDE has made an astonishing difference. Early indications are easily 8-10% increase...with possibly higher numbers. I need to run more tanks on Summer fuel to confirm. But so far, on Winter fuel just keeping the RPMs down alone has made a 8-10% increase in MPGs for me. Wow.

These engines definitely like to be kept at low RPM, this is especially true in a Jeep where you have no aerodynamic flow to help you out.
Author:  BlackLibertyCRD [ Wed Mar 02, 2011 12:42 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
LibertyCRD wrote:
All I know is, there is no question that I burn more diesel fuel at 70 MPH than I do on a 55 MPH highway. My mileage really goes up when my cruising speed is kept down.

Also, the TCM tune from GDE has made an astonishing difference. Early indications are easily 8-10% increase...with possibly higher numbers. I need to run more tanks on Summer fuel to confirm. But so far, on Winter fuel just keeping the RPMs down alone has made a 8-10% increase in MPGs for me. Wow.

These engines definitely like to be kept at low RPM, this is especially true in a Jeep where you have no aerodynamic flow to help you out.


Yes you can be in 3rd or 4th gear at 2,000 RPM and get better mileage then 5th gear because your not running a brick into the wind faster. Lower RPM is lugging the engine
Author:  danoid [ Wed Mar 02, 2011 12:44 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
LibertyCRD wrote:
In all the testing I have done over the years, 2,000 RPM is NOT the sweet spot for the CRD.

All the data I've collected shows that at interstate speed (70 MPH @ 2,000 RPM) I am getting the same MPGs as I do in mixed city driving. In other words, at that point I've gotten past the point of diminishing returns.

For the CRD...45-55 MPH seems to be the true sweet spot. RPMs will be well below 2,000 and the miles per gallon will be at it's peak.

The diesel does NOT like high speed driving. Parkways, freeways, interstate, etc...not good for mileage. I'm shocked that some people are reporting that high speeds give them the best MPGs.

Um, it depends. With the stock engine cal, at some point (maybe around 77 mph?) EGR turns off and BINGO you get a fuel economy boost. So it is possible to see better fuel economy at 80 than at 70 - sometimes. Your mileage may vary. This package was filled by weight, not by volume. Contents may have settled during shipment. Past performace is not an indication of future results. I can't think of any more disclaimers...
Author:  GreenDieselEngineering [ Wed Mar 02, 2011 12:47 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
The lower the rpm the better for fuel consumption as a general rule. 55-60 mph will return the best fuel economy. The Jeep has a very high coefficient of drag and the rolling resistance in the driveline is high as well due to the off-road capability. At higher speeds the wind drag takes over as a determining factor to worse fuel consumption.

This same engine in the Chrysler minivan (Export only) does much better on fuel economy all due to less vehicle mass, front wheel drive and lower coefficient of drag.
Author:  papaindigo [ Wed Mar 02, 2011 2:04 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
X2 the GDE comment. However, the actual mpg optimum speed/rpm for each individual KJ CRD will vary depending on a host of factors under our control such as tire rolling resistence (tire type and psi) and various good (ex. GDE Eco tune) and not so good (ex. F37 tranny flash) and other factors outside our control such as road surface rolling resistance, terrain, and wind speed/direction. All things being equal I suspect that the optimum is the lowest speed/rpms that your particular vehicle can constantly hold on a given road without dropping out of 5th gear lockup. For myself I'm not really into maximum mpg, especially not hypermiling, but rather a nice balance of decent mgp and reasonable travel time. Hence I tend to run, at least on my non-F37 tranny before the GDE TCM tune, at 1800 rpm which is an indicated 62 mph (if I recall the speedometer/GPS speed difference is minimal) which on a flatish Interstate produces right about a hand calculated, GPS corrected, tank filled to the brim figure of 33.9 mpg. About the only other thing I can say is the literature indicates a general significant falloff in mpg once one approaches or exceeds 70 mph but of course that may not be true for an aerodynamic vehicle with a taller upper end drive train ratio.
Author:  fastRob [ Wed Mar 02, 2011 7:14 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
Around 50+mph I can get 30mpg, rural highways, light pedal.
Around 75 I tend get 24mpg.
When I first got my GDE hot flash, forget about it, love to spool that turbo and listen to that auto shift gears.
When i pull out of toll booths I can tell how congested the motor was by the smoke, after a few tolls, it just goes faster and smoother.
Just bought a 7x16xTall 7,000 lb capacity cargo trailer, started out getting around 16mpg, trying to decide whether to use top gear or the next one down, going to go 60mph. The transmission adapted after a few days and mileage seemed to go up but then I took the trailer off.
I use a gps, gray additive(starts good in winter), a good splash of wally world 2 stroke and have 215-85-16 tires.
Probably going to drop down a tire size for towing which screws up speed and mileage calculations but is more stable.
Author:  GreenDieselEngineering [ Wed Mar 02, 2011 8:12 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
fastRob wrote:
Around 50+mph I can get 30mpg, rural highways, light pedal.
Around 75 I tend get 24mpg.
When I first got my GDE hot flash, forget about it, love to spool that turbo and listen to that auto shift gears.
When i pull out of toll booths I can tell how congested the motor was by the smoke, after a few tolls, it just goes faster and smoother.
Just bought a 7x16xTall 7,000 lb capacity cargo trailer, started out getting around 16mpg, trying to decide whether to use top gear or the next one down, going to go 60mph. The transmission adapted after a few days and mileage seemed to go up but then I took the trailer off.
I use a gps, gray additive(starts good in winter), a good splash of wally world 2 stroke and have 215-85-16 tires.
Probably going to drop down a tire size for towing which screws up speed and mileage calculations but is more stable.



If you regularly pull a heavy trailer, you might be a good candidate for a TCM tune. We can dial in the trans to hold 4th gear in the 55-65mph range as this provides the optimum ratio and engine rpm for trailer towing and fuel economy. If you are in 5th gear at the same speeds the trans will downshift/upshift a bit through hills, etc. Plus, when trailer towing the more you are in lock-up the better in order to keep tranny fluid temps lower.
Author:  BlackLibertyCRD [ Wed Mar 02, 2011 8:51 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
GreenDieselEngineering wrote:
fastRob wrote:
Around 50+mph I can get 30mpg, rural highways, light pedal.
Around 75 I tend get 24mpg.
When I first got my GDE hot flash, forget about it, love to spool that turbo and listen to that auto shift gears.
When i pull out of toll booths I can tell how congested the motor was by the smoke, after a few tolls, it just goes faster and smoother.
Just bought a 7x16xTall 7,000 lb capacity cargo trailer, started out getting around 16mpg, trying to decide whether to use top gear or the next one down, going to go 60mph. The transmission adapted after a few days and mileage seemed to go up but then I took the trailer off.
I use a gps, gray additive(starts good in winter), a good splash of wally world 2 stroke and have 215-85-16 tires.
Probably going to drop down a tire size for towing which screws up speed and mileage calculations but is more stable.



If you regularly pull a heavy trailer, you might be a good candidate for a TCM tune. We can dial in the trans to hold 4th gear in the 55-65mph range as this provides the optimum ratio and engine rpm for trailer towing and fuel economy. If you are in 5th gear at the same speeds the trans will downshift/upshift a bit through hills, etc. Plus, when trailer towing the more you are in lock-up the better in order to keep tranny fluid temps lower.



Now this would be a great benefit for the towing I used to do I would cuss 5th gear. I had to get it in 4th at about 1950 RPM went between 60 and 64 mph. It used less fuel and engine ran cooler. I was thinking our towing button on the shifter should have 2 position. Make that three, on, off and 4th. Doh'
Author:  shvlhead78 [ Wed Mar 02, 2011 10:52 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
Just did a 350 mile trip averaged 26.60 on a stock tune CRD 2100 rpms at 73 mph
ORM with a provent K&N filter with the front airbox elbow
removed.
It is a daily driver for the wife and she consistently gets 20 to 21 mpgs mixed driving
so you can see what i see as my sweet spot
Author:  RJM [ Thu Mar 03, 2011 8:30 pm ]
Post subject:  Re: Is there an 'optimal' RPM for mileage?
1800rpm, stock ecm, 27-31mpg with D2.
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