Okay, by the numbers:

1999 E300 Turbodiesel, 140k miles.

Operating temp: Check, 80 C
Oil Level: Check, just below full.
Fuel Level: Near full
Maybe one or two wheels out of balance.

Ambient air temp: 110 degrees F

Running A/C: Wouldn't you.

Will the A/C kill the mileage this badly, or would the out of round/balance tire(s) contribute as well?

Pretty sure that any mechanical resistance will count against me on this one, but I want to see if anyone else has noticed anything this dramatic.

I haven't noticed a dramatic change like that attributed to a/c. Big changes like that usually come from different driving habits, fuel leaks, or anything that's a real drag on the drivetrain systems.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

The AC is a big drag on the drivetrain, but I'm not sure it is worth 4mpg. But at least a couple combined maybe with a heavier foot that tank?????

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

Still driving like an old man. Just seeing different MPG.

I had my car 1999 e300 td since November last year, I'm keeping a log with all my fuel receipts and mileage, and since then my avg. mpg is 25 miles. I always consider this to be normal, is it not?.

Not typical for me.

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I avg about 30.5 mpg typically. Thats 80 miles a day at 70-80 mph and 30 miles a day slow and go 5-30 mph.

At 110F ambient temp if you were in lots of slow and go the aux fans and fan clutch would both be very active and would both affect fuel consumption. Takes more pedal to get going at every light, etc. That could explian the mpg drop in my mind.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

Normal if you drive in the city only, not if driven on the highway.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

This is what I suspected as well. The aux fans and the fan clutch are very active right now. The highway/freeway runs are only 13miles per hop and then we're talking acceleration to 70-80mph very quickly which really sucks down the fuel. We've also had some freaky hairdryer hot headwinds at 20-30mph.

I think that keeping my foot out of it is what's keeping it at only 25mpg rather than a lower number.