I was headed to Modesto today and stopped at the Flying J truck stop to find out what kind of economy big rigs were getting. I asked about 8 drivers of 18 wheelers about fuel economy and the average was 6.5 mpg at 60 mph with a 70,000 load. I still can't understand why I can't do something to my stock 99 dodge pu to get it to do better than 18 mpg at 60 mph when a 18 wheeler that weighs 10 times as much as my truck and has much worse aerodynamics as mine gets 6.5. Seems like I should get much more than 2 1/2 times better economy. My thots are the 5.9 diesel is way overkill for my vehicle and like a previous poster said my vehicle might get 30 mpg with a 4 cyl cummins. The one thing that sturck me in conversation with the 18 wheeler drivers was the RPM they were running, averaged about 1300 rpm at 60 mpg. My truck with 3.54 rear end is running 1500 rpm at 55 mph and 2000 rpm at 70 mph. Does anyone have definitive information on what their fuel economy differences were when they put on an overdrive or changed rear ends? Is there a rear end that I could exchange my 3.54 with that would give me a 3.10 ration or so? I looked at the splitters and overdrives and they are pretty pricy. Like I said previously I tried one of the chips and big improvement in power but no improvement in economy even tho is was advertised as up to a 10% improvement. It seems like if the chip can so dramatically incresase power the chip manufacturers should also be able to likewise increase economy dramatically? (I returned the chip) I carried out careful economy runs to determine the performance of the chip and if anyone is interested in the results I can provide them. I carry a 2000 lb camper much of the time so a VW truck wouldn't work.

 

thought

 

Well if you want to swap in a 4cly Diesel the 4BTA Cummins the engine alone is going to be as much as the gear splitter, plus the conversion and literally, slower acceleration, less able to tow, and 4 banger sound which outweighs 18mpg you are getting right now. Also Big rigs can do 60mph at 1300-1400rpm b/c they max out at 2000rpm, btw they make three to four times as much tq at those rpms then a 5.9L does, "600" or whatever. A semi truck makin 1800lbs of tq at 1400rpm would make 480HP, now you see why they can pull those loads at such a low rpm, they are designed to be that way, and they probably get better economy b/c of Brake Specific Fuel Consumption maybe higher then the 5.9 Cummins, not sure but it could be so.

I think your better off not spending too much money and actually saving money, not in fuel economy but in the amount of money you pay for diesel, If your mechanical enough to change your rear gears, swap in a different engine. Then making bio diesel should be a breeze, 50-70cents on the gallon, who cares what mileage it gets.

 

The cummins sweet spot is between 1800-2100 rpm. With a 4.5" lift, 35" tires, an automatic, 3.54 gears, 4" straight pipe, and a Scotty 2 intake, I can get 16 cruising at 70 mph. I can live with that on a truck this size. You said you carry around a 2000 lb camper most of the time. Are you getting 18 mpg with the camper on? If so, then that's great. If not, then I still wouldn't be complaining about 18. The best I ever did with mine was 25. And that was with an Edge comp in EZ mode, 33" tires, and the above mods (minus the lift). The old 12 valves did get better mileage than the 24's do.

Ripper, the only problem with bio is that you can only run about a 20% mixture of it and #2. Running straight bio diesel would eat up the injection pump cause it's such a good cleaning agent.

 

you'll probably have to replace your fuel filter, but bio is actually better for your injection pump than #2... shouldn't have any problems running B100

 

Then why are people only running a 20% mix? I was told that since it was such a good cleaner that it could possibly eat away the rubber lines if you used to high of a concentration. I also read somewhere that it does have a tendency to gell up a little quicker than regular #2.

 

Wow, I should try to look into it some more, THought I researched it pretty good, just the making part I guess. I think people run B20 b/c maybe thats available at the pumps?

 

Speed is the mpg killer not rpms. Just ask anyone who kept track while driving extended distances with their 5th gear out. Everyone I've talked to with a gear splitter reported no gain in mpgs.

Harder to find B100 in some areas, have to settle with a blend.

 

I have a 6sp with the gear vendors overdrive. It runs 1500 rpms at 70 mph, if fact I seldom rev the engine over 1500 rpms. I get 16 miles per gallon around town and 19 on the freeways, about the same as before I added the gear vendors. It is considerably quieter thou.

 

In response to BigBlue: I get 18 mpg without camper at 60mph. I get 16 with the camper on. What do I do to get 35 inch tires? Does anyone have additional economy information like grzpdlr with before and after something is done to the truck to reduce RPM's?

 

As Rip already touched on, the BSFC on a big truck at 1500 rpm is WAY better than that of our typical CTD's at their cruising rpm........simply slowing the cruising rpm down will do nothing if the end result is you increased your BSFC ratio........in some cases lowering (or raising, for that matter) rpm too much could cause the motor to burn more fuel.........if we compare 12v's to 24v's you will see that the typical 12v has a notably better BSFC ratio than the 24v in the 1700-2000 rpm cruising range.........the 24v past 2000 rpm does however become somewhat better than the 12v........there are too many factors that control BSFC to get into here, but one thing is clear: if our motors had the same hp to torque ratio has a big truck we could expect a better BSFC # but they would be so low on hp that we would not enjoy driving or towing with them.

 

Like Infidel said, Speed and RPMs are the killer, try drivin around at 45 mph with an egg under the gas pedal. If your gettin 18 with the camper thats great. Ive never done better than 16 on any of mine, then 20 empty. On a trip back from Or a while back we had a strong tail wind and I got near 25 on a 95 4x4. now in real times were down to 19 or so. wind resistance will make a huge difference. I think your fighting a loosing battle for 30 mpg. Ever consider a diesel VW camper bus or something like that?

 

johnr9q, if your a 4x4, put a 2" leveling kit on the front and then you can run 35's.

 

The big trucks run at lower RPM's because they have bigger engine parts. If those engine parts get moving too fast the engine breaks down. The same thing happens w/ any engine. The bigger the diesel engine, the slower it's peak RPM's. Check out this link for an example: http://www.bath.ac.uk/~ccsshb/12cyl/

What you need to do to get the best mpg is find YOUR engine's peak efficiency rpm. That is, the RPM's where your truck will be in the highest gear hauling your camper with plenty of power AND getting the best mileage. You can't just run your truck at say 1200 RPM's and expect it to get good fuel mileage because the engine won't be able to burn fuel efficiently enough to produce the power you're demanding. That's when you see the black smoke...if you see black smoke your engine isn't burning fuel efficiently. It's kinda like riding a mountain bike...there's a certain RPM that you can pedal and get the best efficiency out of your legs. You pedal too fast and you get tired faster and/or your feet slip off the pedals. Pedal too slow and it's a nasty agonizing push just to move faster.

Yes, the 5.9L Cummins is overkill even for a 1 ton truck. It's a medium duty engine in a light duty truck. It's the same engine you might find in a rollback tow truck.

Think about things this way...if you wanted to haul that same camper with a gas engine truck your mileage would be somewhere between 6-9 mpg depending on the size engine you get. So you're already getting twice the mpg than ANY gas engine truck hauling the same load...heck, twice the mpg hauling NO load!

 

As you can see by my post that I run a 4:10 gear. I live in Virginia in the mountains. If I try and control my foot I get consistant milage above 20mpg. The normal driving that I normally do with some WOT from time to time, I still get 18mpg on average. I tow a 38ft horse trailer and drive it at least the speed limit or slightly above and get 12mpg. Your truck should do better. Maybe the fuel you are getting is not of good quality. I run at least a 45 cetane or higher as was recomended from a freind that works for Cummins. A lot of the people around here use the 40 cetane and there milage is not as good as I seem to get.