jughead

oops it aint in my sig sorry i do have od

johnr9q

I just got back from Utah and ran another unscientific test on a long flat road that ran in a dry lake bed with no wind. (Read post #10 in this thread for background on the following) I again used my overhead monitor and went 33 mph. In drive out of overdrive I was turning 1375 rpm and got an indicated 29.6 mpg and in 2nd gear turning 2100 rpm I got an indicated 19.5 mpg. This is about a 50% increase in MPG. Looks like lowering my rpm might significantely increase my economy?? Does anyone know where I can get 3.07 gears for my Dana 70 rear end? I called a couple shops and was told that 3.07's are no longer made. Going to an overdrive add on unit from gear vendor or US gear is pretty expensive and I think some effeciency would be lost by adding gears.

maddog55

I went to 34 inch tires on my 99 4X4 5 speed and had the dealer recalibrate the speedometer. Now I run 70mph at about 1900 rpm.
The dealer will tell you it cannot be done. But it can. Just tell the dealer to reset the number of revolutions per mile that your tires do.

www.tirerack.com has tire diameter and revolutions per mile for every tire they sell.

Mine came with 3.55 and 30.8 diameter tires which is about 640 revolutions per mile. When I had 33 inch tires they reset it to about 620 revolutions per mile. I now have 34.5 inch tires. The tire spec sheet says 601 revs per mile. Next year I will but 36 inch tires: the spec sheet says this will be 570 revs per mile, so, the dealer will reset it again.

You might have to tell the guy with the computer what he has to do. But do not let his B S you, it can be done.

Best Regards,

the MADDOG

johnr9q

Maddog: Are you using the same size wheels just with beefier tires? I would think using beefier tires to get the RPM's down might defeat the purpose of improving fuel economy cause it might take additional energy to run the beefier tires??? It would be interesting to see a "scientific test" done before and after to see the difference in fuel economy.

gman07

I'm glad this was brought up, as I have been wondering for a while about it. In an old TDR they said something about BSFC being lowest at peak torque, where volumetric efficiency is greatest. Shouldn't that mean that assuming a certain speed, you will get greatest fuel mileage at peak torque? I haven't studied much, and I was hoping someone like HOHN could see if I am correct in my thinking.

N.Johnson

MAN you guys are thick as a brick sometimes. The same thing over and over again trying to re-invent the wheel. The best thing you can do for mileage is install a boost gauge and pyrometer. You right foot doesn't fuel the motor the computer does. Really, I mean try driving with no more than 6 psi boost and less than 650F pyro. Our motors are rated depending on the model year with a torque rating versus rpm's. Surprise yourself and experiment with that. Of course anything over 60 mph you pay to push your ram through the air which is not the most aerodynamic shape. If your really serious lower your truck and add airdams with smaller tires and heck while your at it, do a gear swap. You will never get your money back in saved fuel anyway. Don't ignore the pure physics involved getting 7000#'s moving. Above all don't go by the overhead readout. Think about how it gets it information. On the cummins isb there is no volumetric sensor for fuel consumption. What the overhead does in my opinion is compare the rail pressure sensor with the map and fuel tank module readouts and average that to the ecm programming tables. Ever notice how the overhead reacts quicker when you reset it? What was that myth someone claiming 38 mpg ....... not on this planet

johnr9q

The 38 MPG was actually documented on a PBS special and was a very tricked out truck owned by McClain Diesel Performance of Phoenix

sled4fun

I thought that these forums were here to discuss topics! I think that this topic of discussion is great.

I would also bet that if somehow through further economy/gearing/aero etc. discussions on this topic someone came up with an economical way to get better fuel economy at the loss of a little power when wanted. There would be a lot more brickheads on this forum reading these posts in a hurry.

Is anyone hear going to get 100mpg........NO! But why not have a little talk on what works to help get better mileage.

POST ON BRICKHEADS!!!!!!!!!!!!!!!!!!!!!! I'm with Ya

FastZilla

There is a ton of science involved but the simple red-neck facts are:

1. Put 35" tires on your truck (A must if you have 4:10s - aka: Off-Road pkg)
2. Keep your RPMs below 1900 (BSFC ~1850)
3. Try to keep EGTs below 600 (Higher EGTs = more fuel consumption)
4. Get some brand of "chip" and put on lowest setting - the stock fuel mapping is more concerned about EPA crap than feul economy - injects fuel late so raw fuel goes out the tail pipe to keep the cat hot & working)

I have been towing approx 7500 lbs around and getting 20-21mpg. at 70mph (1920 RPM). Unloaded I (manually calculated) get 24-25 mpg if I keep it under 68mph (1875 prm) and keep my EGTs under 700 on acceleraton and under 600 cruising down the road - they are normally in the low 500's (Stock air box & exhaust). It's kind of a pain to be fuel efficient to save $5-$7 but it can be done. Most of the spees limits where I live/drive are 75 so it is a real chore for me to do this but it is reproducable, everytime. I saw the biggest improvements with the Banks kit. I can't wait to get the bigger tires - then I can drive 75 and get 20+ mpg. That factory fuel mapping is pure crap!!!!

Haulin_in_Dixie

I would just bet that if you put a four cylinder in place of the six, you would have adequite power and great mileage with the engine bombed a little.

Raspy

Running an engine at it's peak torque RPM is a good way to get the best efficiency. But it is easier with a gasser to see where peak torque is. On the Cummins the torque "curve" is pretty much flat from about 1400 on up, so it is more computer controlled than air flow/camshaft/intake maniflid controlled like a gasser. I don't know what RPM is best on the Cummins.

A certain amount of horsepower must be made to propel the truck at the speed required. A lower engine speed produces less internal friction, but if it's enough to make much difference I don't know. Factor in the unknown compromises made for emisions, such as retarded timing, and maybe higher than neccesary boost.

The few things we can do without any modifications are drive slower, accelate slower, shift early, run narrow radials, and keep your load as light as possible. And maybe, don't use OD unless driving on the highway because OD is less efficient than direct. Then start thinking about advancing the timing, using high cetane fuel, or just get a VW TDI for trips that don't require a 7000 lb vehicle. Another thing is driving the most direct route. This sounds simple and obvious, but every time you take a longer route or miss your turn and have to go back, you have used more fuel for the actual required distance, in other words, your effective mileage went down.

Wetspirit

maddog55

Hey FastZilla,

Cummins has a web site where they offer a calculator and they will tell you at what RPM's you can get the results you want.

I entered 70mph with a manual 5 speed and 34.5 inch tires, for milage. It gave me 4.10 gears at the low end and 4.30 at the mid to high end for fuel economy. I also entered for 36 inch tires at the same speed and it told me 4.30 at the low end and 4.56 at the high end. It will indicate three different speeds - usually 55, 65, and 70 and it will show it in green for fuel economy.

The bad news is, I can't find the site again. If someone finds it, please post it.

Another interesting point is that a couple of friends in Arizona that have had all three generations of cummins engines told me that for economy, the first generation 12 valve was way more economical than the 24 valve. And that the 24 valve was more economical than the new common rails. They say the 12 valve would average 23 to 25 or 26, the 24 valve would average 19 th 22 or 23 on a good day, and that the common rails they now have average 14 to 16 for one guy and 15 to 18 for the other guy. Both told me that they had not seen any thing close to 20 mpg on their 2004 and 2005 common rails.

So what gives?
https://www.dieseltruckresource.com/...cons/icon4.gif
Exclamation
ther MADDOG

infidel

The short of it- higher mpgs have been sacrificed for lower emissions.

FastZilla

This is the reason!!!

I was told by a long time diesel mech that Cummins/DC has a 3rd injection pulse that fires late just before the exhaust valve opens up to send a little raw fuel to the cat to keep it at operating temp - 450-500 EGTs while cruising down the road ait'n enough to keep the cat glowing. I get better mpg's on level 2 with no augments, have more power, etc. Maybe one of the Dodge CTD mechs that lurk around can verify this. But it's kind of like the DaVinci code - a truth of that nature could cause riots, etc. I know I was sure hopping mad when he told me about it. I don't understand how less fuel economy equals a more environmentally friendly vehicle.

Well like I said before, the Banks Power PDA added 2+ mpg. 35" tires will add another 1+ mpg. They are both manditory mods in IMO but now there's a legit reason for them.

xmr

A few years back one of the car mags (car and Driver i think) took a chevy lwb pickup from the 1980's and tried to see what they could do to improve mileage. One of the things that helped was they fabbed a belly pan from aluminum to smooth the air flow under the truck. This probably would help our trucks but it sure would be a pain to work on them.