03 Cummins Fuel Mileage Drop
 

A friend's 2003 Cummins with 47,000 miles on it is experiencing large drops in fuel economy. He previously got 18-20 mpg, now he is only getting 14-16 mpg. He has been to several dealers and they can not find anything wrong with it; no trouble codes either. He has replaced the fuel filter, air filter, and tried using a fuel additive with no success. 47,000 miles seems awfully early for something to go wrong. He also claims his mileage jumps up to 25 mpg when he goes into higher elevation (6500 ft to 10,000+ ft), but that does not sound realistic to me. I told him it sounds to me like the mpg gauge is full of c***. Any ideas, thoughts, speculation would be greatly appreciated.

Thanks
- Teal

Honestly the 14-16 around town is normal. The guys that claim mid to high 20's need to stop watching their liometer and due the math.

Hand calculate! Any new mods?

winter fuel will lower fuel mileage and in town 16 -18 is good dont trust lie meter divide gallons of fuel used by miles traveled :tu:

Those overhead guages supplied by Dodge are crap. They are so far off both ways but some people seem to want to believe them over the accurate mathematical way. All it does is turn people into liars and make them think something is going wrong. Mine has gone all the way from 10 mpg's indicated - I was getting 16 towing - to 28 indicated and I was getting 24 running propane. My wife at least every other week says our truck is only getting 11 miles per gallon and then we go on a 60 mile trip and it says 22. It is so far off and you can't tell them everything or anything.

my 05 only gets 14- 16! a buddy of mine says he gets 24-26! hard to believe there would be that much difference. i drive easy and dont tow alot.

Winter fuel has dropped my summertime 18 mpg Hwy driving down to 14-15mpg tops. Oh I to can now get 24-26 MPG from the over head readout after Dodge worked on my Trans and put in new code to the ECM yet hand calculate it more like 18-18.75 and no more 20.1 mpg that I use to get when it has less mile on it..

I think the 'liometer' is just a tool that says only, "if you continue to drive the way you are driving and no other factors weigh in such as wind, weather, hills, etc,etc, you can expect to get 'x' mpg". We all know that that doesn't happen in the real world, so yeah, ....... do the math longhand.

I have a 03 305hp. auto. I took it on a 100 mile drive babying it and i hand calculated it.
101 miles and 3.99 gallons. You can do the math. Normal highway driving for me is 21mpg.
But maybe check for brakes dragging, possibly dirty air filter with crappy winter fuel could be a cause.

Take it to the dealer have them check the engine! Warranty is great! Use it!