I filled up the 240d the other day and was disappointed to see a fuel economy of 19mpg. The previous fill-up was nearly 28mpg!

Besides driving habits, are there some common mechanical issues that would explain the wild swing? Perhaps somebody siphoned off some diesel, or an accounting error.

I'm thinking the latter...

I did spring for a locking cap on my 300 just to eliminate the first option

That's a good idea. Where did you source it?

Fleabay...

It did ship from Turkey however so there is that.

its been seriously hot lately. AC usage might be a factor.
also, some shady gas stations don't keep their pumps accurate.
tire pressure also effects.
you could check for dragging alternator, ac, steering, etc.

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1977 240D auto w/OM616.916 engine
2005 Matrix 5spd
2006 Prius

https://photos.app.goo.gl/AdP7i8xfCJeFuk5y9

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fused glass pendant http://fusedglasspendant.com

also check for dragging brakes, especially if the soft brake lines have never been changed

__________________

1977 240D auto w/OM616.916 engine
2005 Matrix 5spd
2006 Prius

https://photos.app.goo.gl/AdP7i8xfCJeFuk5y9

glass pendant necklace http://glasspendantnecklace.com

fused glass pendant http://fusedglasspendant.com

It’s hot out. More AC use?

Fan clutch working? More fan engagement that requires 20hp potentially?

Fuel leaking?

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

yeah, check your IP, if its wet, replace the o-rings.

__________________

1977 240D auto w/OM616.916 engine
2005 Matrix 5spd
2006 Prius

https://photos.app.goo.gl/AdP7i8xfCJeFuk5y9

glass pendant necklace http://glasspendantnecklace.com

fused glass pendant http://fusedglasspendant.com

An intermittently slipping odometer? Check the odometer periodically on an Interstate highway using mile markers. Also don't zero/reset the trip odometer unless the car is completely stopped.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Great tips!

I stopped resetting my odometer a few years ago when it got stuck for a few hundred miles. It could very well be sticking intermittently when I'm not paying attention.

good idea...

that would account the 'erratic" as some other solutions mentioned would not

+1 to slipping odometer. This happened on my w126. The slip got worse the faster I drove. In town it never slipped but on the highway it slipped. Had to do with friction or viscous drag in my unit. These are fundamentally different odometers inside but some kind of slip could be easily messing up your mpg.

I found I don’t need mile markers. I just get the car up to 60mph. That is one mile per minute. That means as the 0.1 mile wheel spins on the trip odometer you should click off 0.1 miles every six seconds. I cruise and count off six seconds as the 0.1 flips to 0.2 and so on. Don’t rear end the car in front of you while you’re staring at the gauge.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Weird, I'm having the same issue with my manual 240D. AC no longer works, so, not sure why I'm having such wild swings in MPG (21 vs 26)

Mine is an '81, 240D, manual, AC deleted
A couple years ago, I put Firad nozzles in matching number injectors and had Greazzer clean and pop set them. Mileage went from 26 to 22. I sent him another set with older Monarch nozzles. I installed a 3.46 diff and speedometer at the same time. I have been through only one tank of fuel, but I got 28. Mostly in town driving. I spend a lot of time in 2nd and 3rd. i am wondering if advancing the injection timing a degree or two would make a difference.