5.9 Cummins horrible fuel mileage

[Fumminsman]

HellI I have a 2003 Dodge Ram 3500 with the 5.9 Cummins. This truck is stock at the moment except I do have a smarty touch on it. This truck has 240000 I bought it with 218000 about a year ago with a bad transmission. I don’t know much history on it but I imagine it has the original injectors. It has a 3” lift with 35’s in stock 17” wheels. The gas mileage is horrible like 10.5-12mpg doesn’t matter how I drive it. It seems to get worse with highway driving. I just drove from California to Georgia to Colorado and back to California I was only getting 11mpg on average. Before anyone says anything this was hand calculated not from the lieometer. I’m only getting 370 miles or less each tank. I have friends with the same truck same setup getting 17-22mpg. I also have an early 04 Cummins getting about the same (can’t really compare the two since it’s in an 06 f250). I’m curious if anyone on here has any good ideas as to why. I chose this site because in my experience the people on here seem to know more. My rail pressure seems ok according to the smarty the desired and actual are very close. Although compared to my dads 06 that I just put 100hp xergy’s in its lower he was running 17000-18000 cruising at 60-65 where as I’m sitting at 13000-14000. His truck is stock except for that. Before anyone says anything the filters and fluids are new and frequently done. Does anyone have any good direction or advice on this? One of the reasons I bought a Cummins is because they tend to get pretty damn good mpg. Thanks any help would be appreciated. 

Edited August 30, 2020 by JAG1

[dripley]

Have you re calibrated the truck for the over sized tires? Those and the 3 inch lift dont help mileage. 

[Fumminsman]

I’ve already had that thought the speedometer is off by 4-5 mph at 60-65 mph. So the math doesn’t add up it might add 60-70 mikes per tank. That’s still very shy of the 600+ miles most others I know get per tank. 

[dripley]

Are the others lifted and on 35" tires? Are the gear ratios the same? Different injectors, tuners? They all make difference. 22 mpg is darn good mileage. I have only seen that mileage a couple times on mine. Mine will normally seem19 to 19.5 on the interstate, maybe 14 around town with a good bit of stop go. Of course mine is a 2nd gen and I might be comparing apples to oranges.

[Mopar1973Man]

19 hours ago, Fumminsman said:

It has a 3” lift with 35’s in stock 17” wheels.

There is the problem. 

 

19 hours ago, Fumminsman said:

The gas mileage is horrible like 10.5-12mpg doesn’t matter how I drive it.

Final ratio is too low and the vehicle is standing up in the wind extra foot now. 

 

The only thing I could suggest is either...

1. Return it all back to stock and remove the lift. Reset the tires back to stock to gear the gear ratio correct. 

 

2. Change for 4.10 gears and this will cost a pretty penny last bid I seen up in McCall ID for was $4,000 for both axles to be swapped to 4.10 gears.

 

Basically you have a final gear ratio like 3.20:1 to the ground and adds huge amounts of engine load and heat. 

 

Oh just to show you what tires can do. Now your on 35 inch tires and empty barely getting 10 to 12 MPG. Now I run a 30 inch tire 245/75 R16 no lift. I can drag my 31 foot Jayco RV which scales out at 17,300 gross combined pounds and still getting 14 MPG. So you added a 3 inch lift and 35 inch tires ruined the final ratio and create a drag equal if not worse than my RV being towing behind you virtually. 

 

I'm only 18 feet less than a normal semi truck and trailer. 

 

Optimal final ratio to the ground should be 3.55 to 3.73 to 1 to the ground. Below 3.40 ratio to the ground is going to increase engine load and consume more fuel.

• 66 MPH = 2.000 RPM (Typically see 20 MPG empty)
• 82 MPH = 2,500 RPM (Hi Marked 21 MPG empty)

Being you have a Smarty I would opt to get the unlock dongle and get to tuning that software. 

 

 

Edited August 31, 2020 by Mopar1973Man

[Fumminsman]

I would agree with you on the tires but like I said I know several people with trucks the  same as mine And they get at least 16mpg. As far as the lift a 3” is basically a leveling kit plus a half inch with 1” blocks in the rear. Now on average of people I know with these trucks they get good gas mileage 16-19 mpg at least but I have talked to people that have a completely stock truck that gets shitty mileage. Maybe it’s just the luck of the draw but I should be getting at least 14-16 mpg 11 mpg isn’t purely from a small lift and 2&1/2” taller tires. I mean my dump trailer with 13000 lbs in it only drops my mileage by about 1mpg maybe 2. I’m probably going to put twins on it to help with mpg and egt’s just not sure what I’ll gain mpg wise hopefully I can pick up 3-4mpg. If all else fails at least it will have enough power to make me feel better about 11. Lol

[Mopar1973Man]

Does effect the way they run.

 

Being our truck are mechanical pump still (electronic controlled) and the injectors are mechanical all you need to have is wore our VP44 and wore out injectors and most likely will not get good MPG numbers. Injectors are only good for about 100k ot 150k at best. Then VP44 are on average about 200k to 250k with most of us. Now take a injector that is like my stock OEM from new at 150k miles dropped to 260 bar and dropped nearly -4 MPG. Just because the injectors where popping at 260 bar. Still ran great but spray was too thick. Not a thin mist but bigger droplets. Now most of the droplet is wasted in smoke production. Then if you have VP44 that is wore out and timing piston is not working correctly then MPG's is lost too.

 

Just remember more boost means more fuel being burned. My average boost at 65 MPH is 2 PSI (550°F to 600°F EGTs)... My average boost at 80 MPH is 5 to 7 PSI (750°F to 800°F EGTs). I suggest you reduce your working load first. The EGT's will fall as well. Drag causes boost to occur reduce the drag and the boost will fall and so will EGT's. 

 

Even towing my EGT's are controlled and never cross 1,200°F with cruise set at 65 MPH that includes grades up to 6 and 7%. No problems. Still boost averages very low at 8 to 10 PSI on the flat towing. Made less than 25 PSI towing on grades typically.

 

 

Edited August 31, 2020 by Mopar1973Man

[Fumminsman]

Yeah injectors are part of what I’m leaning towards I was thinking of putting in 100hp xergys along with the twins. I’ve read lotsssss of forums about fuel mileage everyone seems to lean towards the bigger tires and driving habits and lifts  and all sorts of other things that just don’t hmm add up to me. I truly believe it’s something else like injectors or I guess possibly the pump. I was just looking for someone that I believe knows what they’re talking about to confirm this. So thanks because for confirming my thoughts. I truly believe this is something that can be fixed be replacing something. Although I won’t lie swallowing $3000 in injectors isn’t what I want to do but if I can gain back 5+ mpg it’s worth it. I don’t know what the egts are running at in this truck I haven’t put a probe in  It yet. But I can tell you my 06 f250 with the 04 common rail would never stay as cool as yours with a load on a grade like yours. It towing this dodge home from Oregon on a 6% grade fit 6 miles I hit over 1900 degrees on the egts and overheated the truck at maybe 45mph. That’s the reason I’m going twins it Lowers your egts drastically and allows your to run a pretty aggressive tune even while towing. The real problem is I’m not supposed to be messing with this truck it’s just supposed to be a work truck lol. The Fummins is supposed to be me play toy and project but it requires more money than I have to spend right now to do what I’d like to. 

On another note something I didn’t mention in the bio of my truck this is not a high output model. Although since I have a tuner I don’t believe that makes much difference. But wanted to put that out there 

[That Guy]

That egt is insane. I have pulled my 97 12v on a 40ft gooseneck with a bunch of other stuff with my 2012 6.7 (grossed 25k lbs) and got 11.5mpg on my trip and never broke 1400* (emissions can run it a lot higher). And that was up Mont Eagle Mountain with a 6% grade. I dropped to 45-50 and was on the floor, never came close to overheating.  My 2012 has 285/75/16's (33").

 

Neighbor with an 03 Cummins CR on 33"s gets about 14mpg hauling 20-30k lbs of hay in the flat lands.

 

Your mileage is really bad, even for a lift and bigger tires. Agreed, injectors are primary suspect. Also..... a clogged catalytic converter could be it, Dodge has a really spotty history with putting cats on these trucks so yours may not have had one. My 12 valve did. Neighbors 12v does, his 03 CR doesn't. 

 

Fix the mileage before throwing twins at the problem. What's your power goal, 500 at the wheels?

[AH64ID]

@Mopar1973Man no VP on a 03. 

 

 

@Fumminsman there is something wrong for sure. Have you done a compression check? Calibrate your speed, as it's pretty easy with the Touch. What settings on the touch? What are your cruise EGT's like? Cruise boost? How about WOT boost/EGT's. 

 

Even your 04 needs some help, 1900° isn't good. Either you have your tuning really messed up or something is going on, I'd figure that out before getting twins as that might just be a bandaid. 

[JAG1]

Stuck brake caliper(s)? Check for heat at each wheel after a short run.

Edited September 1, 2020 by JAG1

[Fumminsman]

Yes my 04 cr I the Ford does get to some insane egts it has 150hp nozzles on it and a tuner pretty much stock besides that. Yeah 0-60mph wot it will hit 1600 easy. But that truck has other issues too. I think shortly after I put it in an injector got stuck I changed them but was too late now it has quite a bit of blow bye it needs to be rebuilt. That project is on the back burner for the moment. My 03 I think probably needs injectors cause this mileage is killing me. To answer the question I’m looking to get 500-600 hp nothing too crazy just a 77mm over the stock. I’m not supposed to be playing with this truck but I can’t help myself. For the 04 I already have a s364 and s475 kit but I need to fab the piping. Im looking to get 900+hp out of that. But that’s a lot of work down the road. I’m just getting a cheap on3 twin setup for fun. I just want to put a little pep in my 03’s step I really like the lower egts and drive pressure you get from twins. But looks like I’m doing injectors now too man these trucks get expensive. 

[AH64ID]

150hp nozzles and a mild tune is a lot of fuel on a CR as even the smallest tune adds fuel over stock. You should look into custom tuning. 

 

In many cases the power desired can be obtained with less than stock duration, which means a quicker cleaner burn and plenty of power. My previous 05 dyno'd 425/850 with ~80% stock duration and BBi Stage 1's. It had a lot more power left in it, still below stock duration, but it was a tow pig. I could run 70 mph up a 5% grade at 7K feet on a 75° day at 20K GCW, in 6th gear and not break 1250°.  

[That Guy]

39 minutes ago, Fumminsman said:

 But looks like I’m doing injectors now too man these trucks get expensive. 

Understatement. 

 

A 77mm should make a nice cool 600hp.

[Fumminsman]

Yes tuning on my 04 is on the list but like I said I have bigger plans for it don’t want to waste money on custom tunes that I’ll only use temporarily. I’m still fighting to get the 5r110 tuned and working right with the Cummins. The hardest part is getting parts into California. For the turbos I have to have them sent to a buddies in Nevada but he also has to pay for them because the shipping and billing have to match. It’s a pain in the *** I can’t even get the 100hp xergys sent here. Now I have to figure out how to get those probably have my dad in Colorado order them then send them to me. I hate California I can’t wait to get out of here next year.  I’m hoping not to make much more than 600hp with the 03 I don’t think my Tranny could handle much more. 

[Fumminsman]

Just curious have any of you ever tried raceteck or exotic head studs. Citicorp has a lifetime warranty just like arp and raceteck doesn’t but they say they’re good for 240000 psi. Any thoughts?

Don’t know why spellcheck turned xotic into Citicorp but it did lol

[Fumminsman]

UPDATE: so I installed the 100 hp xergy injectors that got me another 2-3mpg then I installed the compound turbo and put in the stock turbo from an 05. That got me a little more so now I’m getting around 500 miles per tank instead of less than 400. So everything was looking ok still not anywhere near the mileage I should be getting. But then I drove out to Georgia and picked up my 28’ enclosed trailer (5000lbs). Now I’m getting 8-9mpg on the highway which is less than I got the last time when I took the truck stock 10-11mpg. Wtf is going on this is ridiculous. Does anyone have any ideas this is driving me nuts!! I’m about to light this truck on fire and collect the insurance money. I bought a Cummins because I wanted the power reliability and FUEL mileage. Thanks for any help

Also I seem to get about the same mileage no matter what tune I use. I haven’t put it on anything above 5 though on my smarty touch

[LiveOak]

This might help.

 

5 Ways to Get Better Fuel Mileage

 

You may want to check and clean or replace the IAT sensor as well.  

[Mopar1973Man]

16 hours ago, LiveOak said:

ou may want to check and clean or replace the IAT sensor as well. 

 

Myth..

 

I quit cleaning the sensor and just check the sensor again the ECT at start up if matching the sensor is working. Below 80*F there is about 3* to 4* advancement of timing. Warming the IAT the better for MPG's. No need for cleaning or replacement unless the temperature is way off. Typically I see about +40*F over outside air. Then IAT and Fuel Temp typically run as a match pair. 

 

I'm still running my OEM IAT it has not been clean in hundreds of thousands of miles but the temperature is dead one the money.