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Go Back   Dodge Cummins Diesel Forum > 2nd Gen. Dodge Cummins 94-98 12V Forums > 94-98 Powertrain
94-98 Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect...NO ADVERTISING

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Old 11-22-2011, 01:34 PM   #1 (permalink)
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Low MPG

I'm getting about 15mpg on the highway at between 75-80mph with the truck in my sig. It turns right at 2000rpm at 75mph. I read somewhere that valve lash can have an effect on mileage so I have that on my list. Looking for some other ideas on how to improve it. How much vacuum is normal for the little air filter indicator? My air filter appears clean, but it is registering some resistance.
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Old 11-22-2011, 02:34 PM   #2 (permalink)
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I'm getting about 15mpg on the highway at between 75-80mph with the truck in my sig. It turns right at 2000rpm at 75mph.Looking for some other ideas on how to improve it.
That is simple enough... SLOW DOWN!!!

Seriously, the previous owner of the truck I bought told me he got 18 mpg. On the way home (with lots and lots of idling at truck stops) I got 23.5 and 26 mpg. The difference? I was driving 54 mph at 1500 rpm he was driving 70 or so.

Speed and rpm makes a huge difference in economy. I think I dropped to around 18 mpg when I drove at 70 mph when I was almost home (only 200 miles left)

Lower rpms gives better fuel economy. You could add an overdrive or install gears like 3.23 or 3.07 for better economy but worse towing power. As I understand it the 1st gen offered 3.07 gears for the cummins. I expect it was only for the automatic but could be wrong. You could also put on taller tires and a new speedo gear to be accurate.
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Old 11-22-2011, 03:02 PM   #3 (permalink)
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Thanks for the input. I'm not sure what gears I have, I'll have to look. I am planning on going to a taller tire eventually, but I want to wear out the tires I have first.
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Old 11-22-2011, 03:36 PM   #4 (permalink)
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I would guess you have the same 3.55 gears I have. If you tach just about 1,500 rpm at 54-55 mph in OD that should be right. You should see a sticker under the hood telling you the gear ratio.

One other thing, if you do not have an air dam under the bumper add one. They are cheap and I have read they add 1, maybe it was 2? mpg.

I have been told that a BHAF (Big Honkin' Air Filter and a 4" ( or bigger) exhaust help fuel economy. Have not done it yet myself so I have no comparison.
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Old 11-22-2011, 03:37 PM   #5 (permalink)
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I'm getting about 15mpg on the highway at between 75-80mph with the truck in my sig. It turns right at 2000rpm at 75mph. I read somewhere that valve lash can have an effect on mileage so I have that on my list. Looking for some other ideas on how to improve it. How much vacuum is normal for the little air filter indicator? My air filter appears clean, but it is registering some resistance.
Sounds just about right for that speed and being an automatic. My last 12V was a 5 speed and pump turned up and timing advanced, 4 inch exhaust no muffler or cat. It religously got 20mpg even on shorter trips. The current is same truck bone stock but auto and it gets 15mpg on the same trips. Out on the highway at 55 or so mph it gets almost the same as the 5 speed truck. I saw 23 mpg on a longer run. The stick got a best of 24 mpg. Both trucks geing geared 3.54.

Mind you the current truck has a fuel leak I can't seem to find. If it had 4 inch exhaust straight through and the timming advanced it might be close to the 5 speed in mixed driving. The stock auto soaks up a lot of efficiency. I've heard an upgraded tq convertor helps with the power loss that is lost through the transmission.
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Old 11-22-2011, 03:38 PM   #6 (permalink)
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I'm getting about 15mpg on the highway at between 75-80mph with the truck in my sig. It turns right at 2000rpm at 75mph.
i run around 65 and get 20 compared to 18 around 70
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Old 11-22-2011, 03:41 PM   #7 (permalink)
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As I understand it the 1st gen offered 3.07 gears for the cummins. I expect it was only for the automatic but could be wrong.
your right it was for the autos because of the 3speed tranny no over drive
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Old 11-22-2011, 05:42 PM   #8 (permalink)
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Adjust the timing up to 16-17*. Reset valve lash. Verify the fuel presure and increase it to 40-45PSI. Make sure the fan clutch is disengauging as they tend to stick robbing power. Air the tires up to close to max. Make sure the rear drums are not dragging. Forget about the air cleaner filter minder and change the filter if you think it is dirty. I could pull mine into the red on one WOT pass on a brand new filter! Cange the fuel filter if it has been awhile. Verify that the fuel heater isnt sucking air and the filter screen is clean. Get the injectors popped if its been awhile.
My twins truck runs up the interstate at 22-2400 RPM then I idle it through alot of traffic. I have avg 18.4 MPG for the last 5 months with dieing injectors. Then again it is a 6 spd which normally get worse milage than an auto truck. My OD gear is .79.
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Old 11-22-2011, 06:00 PM   #9 (permalink)
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Thanks for all the help guys! Now I have a nice list of things to go check and try. This forum rocks! :-)
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Old 11-22-2011, 07:26 PM   #10 (permalink)
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Bumping the timing up to 15-16 will give you better mileage and more power.
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Old 11-23-2011, 06:24 AM   #11 (permalink)
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Make sure fuel pressure is in spec, too low will cause a loss of MPG and power.
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Old 11-24-2011, 10:58 AM   #12 (permalink)
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Found the tag. Gear ratio is 3.55. Have to do the KDP fix still, plus the timing cover is leaking, so I'll plan on doing those fixes and bumping the timing at the same time. I'll have to get a fuel pressure gauge to check that.

Just another thing I was noticing; My truck makes zero smoke, none at all even at full throttle from a dead stop. I'm just fine with this as my goal is not to make smoke, but it got me wondering. I remember driving an old coworkers stock 12 valve a few times, and it would put out just a little smoke when you first gave it some heavy throttle, then clear up as soon as the turbo caught up. Does this maybe suggest I'm not getting enough fuel or low pressure? I'm going to get a gauge to verify anyway, but just running a theory by you guys in the mean time.
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