My 90 sdl is getting 21 mpg. When I last checked 3000 miles ago it was getting 24. My driving hasn't changed and I use diesel only to start and warm up so it can't be winter fuel. Compression is 375 +/- 15 on all 6 cylinders.
Only change I notice is that there is a little smoke at idle. Nothing I can see in the rear view mirror. Doesn't smoke on heavey accel. either. I do see some smoke at night when there is a car behind.
I am thinking the car is running 'rich' in gasser terms. I am afraid to mess with the alda because the car runs great. Smooth and ~ 11 secs to 60.
Any ideas?

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82 300D....went to MB heaven
90 350 SDL....excercising con rods

I get about 25 mpg at about 70; 21 mpg at 80. Just put 3000 mi in a trip to Tampa and back, total mpg at 24; diesel ranged from 3.09 fla to 2.25 in VA.

I think all diesels smoke. At night I see smoke too; I like to floor it and kill bugs....

burned less than a qt of oil too. 115K on the car.

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71 220D 169K wrecked
83 240D 118K sweet 4 speed
91 350SDL (one of the 60% good engines) 156K
84 300d (loaner to my sister) 189K
79 300SD (partswagen)
86 420SEL partswagen
70 220d (partswagen)
68 280s GASSER!!! under construction now
85 300sd 310K miles winter beater car retired
93 300d 2.5 turbo 168K wife's car
83 280SL euro 5 speed 155K
69 250S newest project 54K

Dirty air filter maybe? Brake dragging? Carrying anything heavy? Tires not inflated properly? Winter fuel?

It could be a few things.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

New air filter.
Brakes spin freely.
Extra 26 gallons of vo and corresponding AL tank.
Tires at 30 psi front 33 rear on 225/55/16.

Could a change from 205/65/15s to 225/55/16s (16 rims weigh less than the stock 15s) and 26 gallon tank cause a 10%+ drop in mpg?

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82 300D....went to MB heaven
90 350 SDL....excercising con rods

I don't know. It seems to me that once you start talking about WVO YMMV, no pun intended. Generally WVO has a little less energy content than an equivalent volume of diesel, and I'm not surprised your mileage is it the low twentys given that you also have a separate tank you haul around. Check one of the online tire size calculators to see what type of change you should expect in odometer readings based on your tire swap.

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Tjohn

82 300 SD
77 450 SL (gone)

Could be that they're starting the winter blend? I dunno if that accounts for 3 mpg though.

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1985 CA 300D Turbo , 213K mi