johnr9q

I read with interest claims that a fellow from Arizona got 38 mpg out of the Dodge Cummins pickup that was highly modified to improve fuel economy. I wonder if lowering RPM helped him to obtain better fuel economy? I talk to big rig drivers who say they get 6 mpg pulling 60,000 lbs and at 65 mph they are running 1200 rpm. (6 mpg seems really good for all that weight) I have had experience with overdrive transmissions over the years (mostly gas) getting improved fuel economy over the same vehicle w/o overdrive. I have 3.54 gears in my 99 cummins pickup and am considering putting in 3.10 if possible to see if I can improve economy. Does anyone have specific numbers on MPG before and after they experimented with this concept. Thanks, John Robinson Sacramento

WUnderwood

getting 38 mpg is not happening, well, it could, if you drive 20 mph.

lowering your RPM's will help MPG. if you have 4.10's and you drop down to 3.73 you will improve your fuel economy if you arent towing, on the highway. most engines get the best fuel economy when the RPM's are lower, because the motor is not working that hard. most say that the gear swap improved their MPG by 1 or 2. you can also install a Gear Vendors Overdrive that bolts to your transmission, that also helps, but it is pretty pricey

I am going to switch from my 4.10 to 3.73 in the future to improve my MPG because I dont tow at all, just waiting for the $

hope this helps

Raspy

Lowering the average RPM of the engine is bound to help with mileage. It should help because there would be less engine friction. But it may be impractical or no help if done the wrong way. You are not planning to, but a lot of people seem to try putting on huge tires and think they will get an increase because of better gearing. But the rolling resistance and wind resistance increases so much that mileage goes down. Overdrive is another thing that may help but only if it is correcting a problem that is bigger than it causes. Whenever you run through another set of gears you are robbing power. The best would be a straight through tranny with the final ratio high enough in the first place, not an overdrive. It seems like there is only so much you can do with such a heavy vehicle, with so much wind resistance, and with so many built in power robbing conditions. Then there is speed and fast starts, huge contributors to poor mileage.
The bottom line is, 3:10 gears are going to be a pain to drive with and expensive to install. There are other things you can do that will help more (like slowing down), and there is no magic bullet that will suddenly make a huge difference. I've been wondering if the combination of a different turbo, a different camshaft and different fuel timing might make a significant difference. How much mileage is wasted to keep the exhaust clean?

Wetspirit

Haulin_in_Dixie

As has been mentioned, it depends on the situation. Take a different approach. There is a sweet spot, normally 1700 or 1800 for economy. IF you can gear to achieve that with no overdrive (including the one in the trans) you will be doing about what you can. Then drive at a slower speed.

If you run heavy, the low rpms may or may not be the key. If you lug it down to keep with the hills, sometimes a higher rpm with the engine loafing as opposed to lower and pulling hard will be a better bet. The truck has to have adequite power to pull the vehicle at the speed and on the terrane that you are driving.

Each gear set costs economy. Forget the figures but something like 2%. For top economy, geared for the speed you drive in a direct gear is best. In that case, without actually checking the speeds, the 310 and driving in fifth on the 5600 would probably do good.

Todays big trucks get better mileage than before. I get 7 1/2 to 8 pulling a six car trailer with 500 hp N-14. With a three car trailer it is 8.7 to 9 and bobtail it gets 11. The tractor is 13.6 high and tandem axle.

RAMRODD

I know going 1 gear higher and lowering my rpms to 1800 on my tractor really helped. (8.3 cummins) last year I averaged .76 gal per acer this year so far I am at . 59 everything is the same except for lowering rpms and gearing up.

I think the same would work for our cummins trucks get your rpms in the beginning of the peak torque range. I would think that would give peak economy. That has to be why the big rigs run that way, in my opinion anyway.

infidel

I don't believe rpm makes much difference at all in fuel economy, just ask anyone who has an aftermarket overdrive or driven a long distance with a missing 5th gear.

Way it was explained to me:
energy used (fuel) = speed + mass (weight) + distance traveled.
No rpm in the equation.

robert chilton

I agree with infidel. My truck has proved to me more than once that it gets better MPG at 75 than 70 MPH. The higher speeds require the right amount of RPM to pull in the MPG. At slower speeds a higher gear that isnt lugging should be the hot ticket. I drive my truck back and forth to work and rarely use 5th gear running 50-55 MPH and average mid 18 MPG average on a 6 mile intown drive with 2 blasts up to 55.
Best advice is play with your truck a little and find she likes. Just do the runs all week and fill up before trying the next range. Once you have a few speeds the motor likes try it again to verify the data. Then all you have to do is adjust your driving to that. 450-600 degrees on the pyro with little boost should be where you find your sweat spot for MPG.

johnr9q

I definately think there are things that can be done to the dodge cummins to significantly improve fuel economy. I bought my rig with the thot of fuel economy in mind (a diesel engine is more efficient than a gas engine. I actually rented a diesel 98 and a gas 98 before I purchased my rig and ran a 100 mile course with each and got 5mpg better fuel economy so decided to purchase the diesel) I didn't care much about the power aspect but it seems that most people on this web site are more concerned about "bombing" their rigs. Especially with the cost of fuel going up I would love to see more and better info on increasing fuel economy. People often respond to my request for info on increasing fuel economy with a formula of weight vs speed equals MPG or some other such info. I believe that if a Big rig pulling 70,000 lbs can get 6mpg at 60 mph (I actually went to a truck stop and talked to a number of big rig drivers and this was their average) then my 8,500 rig (includes my 2000 lb camper) should get significantly better than 14 under the same conditions. Also people with diesel pushing motor homes that probably weigh at least 30,000 lbs get 8 to 10. Wetspirit (previously in this thread) talked about improving economy by experimenting with Different turbo, different camshaft and different fuel timing. These are the kinds of things that I'd like to have info on that people have experminted with and have specific fuel economy results before and after results. For example, I was told by people on this forum and by the manufacturer that I could expect approximately a 10% increase in my fuel economy if I installed a True Torque Power Module from Diesel Dynamics so I purchased one. These are the results of 3 economy runs I made over a roundtrip course of 89.3 miles: 1) without the power module 17.82 mpg, 2) With power module set for timing only 17.82, 3) with power module set in #3 position (most agressive) 17.00. Needless to say I returned the module within 30 days and got a full refund. I tried to discuss my economy results with the representative and find out how they got 10% but he didn't seem interested, said most people were only concerned about the increase in power and it sure did increase my power but in my opinion my truck has plenty of power, stock and personally I'd sacrafice power for better fuel economy.

HOHN

True, but RPM matters because it determines where the engine is operating on its BSFC curve-- or HOW EFFICIENT it can make the energy needed.

RPM doesn't matter as much on a diesel as a gasser because of the variable air:fuel ratio.

As an experiment, I'd like to test my MPG once by running 1800 rpm in 5th gear (about 45mph), as I suspect I'd clear 25mpg like that.

I *have* gotten over 25mpg driving at 52mph in 6th gear, which is ~1300 rpm, and easly lugging if you overload the engine.

jmo

johnr9q

I ran a little unscientific test yesterday to determine what effecgt lowering my RPM whould have on fuel economy. I used the MPG calc from my overhead monitor (it is fairly consistant but usually indicates 1.5 to 2 mpg higher than hand calc) I just ran the test long enough so that the overhead monitor settled in on a specific MPG. I ran at 55 mph first in overdrive and then out of overdrive to see what the diference in RPM makes in fuel economy. I know this may not be completely valid cause in overdrive the trans is "locked up" and out of overdrive it is operating thru the torque convertor which is less efficient. I plan do do another unscientific test and run at about 35 mph and use 2nd and 3rd gears and see what the difference is there cause that would be going thru the torque convertor in each gear. The results were out of overdrive I was running 2200 rpm and got 22.5 mpg and in overdrive I was running 1500 rpm and got 27 mpg. Looks like possibly reducing the RPM gets better fuel economy?? As I said This is very unscientific. I would be very interested in knowing if people have purchased after market overdrives have scientific data on before and after overdrive and it's effects on fuel economy.The following is one suggestion on how to obtain "good scientific information": You need to determine the hand calc MPG first in the stock mode and then with the overdrive engaged or smarty set for best fuel economy or whatever you are testing to determine valid before and after fuel economy figures. It takes time and money to really test properly. You need a course that is at least 100 miles and be a round trip course and one that you run at a time of the day where traffic will not slow you down. (the course I run is flat but ideally two or more courses would be used, one that is flat and one that has grades) Also best to run the entire course a number of times at various speeds, say 55, 65 and 75. You need to fill up at the same station at the start and finish of the course and at the same pump. You need to insure that you fill the truck tank to the same level each time. You need to run the before and after course as close together in time as possible and insure that the same conditions exist (wind, temp etc) The information I provided in a previous thread on the True Torque Power Module was obtained using the above guidelines. As I said, it is not easy to really check out a mod but anecdotal info info is not adequate.

Haulin_in_Dixie

for economy, how about going to a four cylinder Cummins in place of the six cylinder. Weight savings and more economical engine.

Buffalo

TST is working on a box that is going to be "dedicated" to economy. Couldn't say anything about it.

Dennis Hurst, the head man at the Cummins Mid Range Engine Plant in Columbus, IN., in a seminar, said that the "sweet spot" for the 3rd Gen. engines is in the 1700-2000 rpm range. He said for best economy, stay under 2000 rpm. Less is better.

With my '91.5 W-250 pulling a 25' travel trailer (7000lbs) across the flats of northern NM, into the Panhandle of TX & on into OK, if I maintained 55-57 mph, I was getting 15 mpg. 62-65 mph & I got 14 mpg. 67 into the 70's & my mileage went down to 13 mpg.

I think a lot has to do with wind resistance, also. It seems to take more power to cut through the air faster.

Joe F.(Buffalo)

Raspy

Wind resistance increases exponentially. Remember, it's an acceleration. The air has mass and must be moved out of the way. Just like accelerating any other massive thing, the quicker you make it change speed, the more power it takes. A little faster vehicle speed means a much higher wind resistance.

John

jughead

if one is not pulling why would 3.10s be hard to drive. i tow very little most gross is about 15k. drop it out of od in hills. back in od in flat country. 3.07 axle and LOVE it. get better mileage towing that weight than my sons 7.3 ford does empty. look at my sig. it aint a world beater but a lot of gassers have looked at tail lites only. had this set-up for about 7 yrs.

robert chilton

The reason the 3.07s are working for ya is that you dont have an OD gear. 3.55s with a .75 od put cruise RPM in the same range.