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Discussion Starter · #1 ·
I have an '03 2500 Cummins.

When I first bought it I got 17.4 mpg around town consistently. Highway was about 19.6 and towing a 5500 lb trailer was about 16.5. No variation at all, it was rock solid and consistent.

That was then, and this is now. As it's been aging (currently 70k) the highway mileage is increasing, but the city mileage is dropping. Also it's becoming increaingly sensitive to speed and weight. Going from say 55 to 70 drops it a good 2 mpg where before it was rock solid. Also adding just moderate weight such as a couple of passengers drops the mpg by at least 1 mpg.

Now around town I barely get 15 mpg, but on the highway it's up around 23 (55 mph).

Anyone make sense of this?
 

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I have no idea if this could be your problem, but I’m going to take a shot in the dark. When was the last time you changed all your fluids? Trans, T-Case, Diffs could all have older well worn oil that isn’t allowing proper lubrication, thus reducing your mileage.

I switched all my fluids over to AMSOil at about 5,000 miles in my truck. They make probably the best synthetic oils out there. Been using it in my vehicles for awhile now, and love it. Ya it costs a little more, but if it extending the life of them, its worth it.
 

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Also, if you are going by the overhead, don't believe it. I vary anywhere from 2 to 4 MPG from the overhead and pen and paper.
 

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Discussion Starter · #5 ·
I had the dealer do all the fluids and maintenance at 40K. I have a dealer that's a pain with respect to warranty, so I had them do it to avoid a problem.
 

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Hmmm. The only other things I can think of is, maybe a dirty air filter. Low tire presser? You driving with a habits changed maybe?
 

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That is not too good at all. My truck has 92K on it and I get 23 on the hiway empty running 70. I get 19-21 in town empty. Towing 5500-6000 pbs drops me down to around 17. Of course I drive like a 80 y.o. with bad knees walks.
 

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Remember to minus about 2 MPG for the 4x4 verses 2x4. The 4x is heavier, higher and you are getting more resistance from the transfer case whether engaged or not.
 

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I learned that bigger tires and an performance module dont help at allllllll. I get a whopping 13.5 - 14 mpg. But its worth the black smokescreens that I can leave all over the place.. hahahahahahhahahahaha
 
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