well heres my question i have a 82 300sd that gets great milage aroud town but today i went to albany from binghamton ny. at about 75 the whole way and got terrible milage at about 18 mpg normal around town i get 25 what are some things to look for? the guy behind me said there was a constant light smoke the whole way.. it always starts good ,shakes at idle (common problem) its got 187000 miles great power uses very little oil any ideas would be great i would hate to have to get rid of it ugly as it is( work car ) i checked the IP it was right on as for the cam timing it is off but not that much PLEASE HELP!!!!!!! THANKS

How much is cam timing off?
When was last fuel filter change?
When was last oil change?
When was last diesel purge?
When was last valve adjustment?
What is operating temperature?
Do you keep the A/C on?
Do you drive full throttle?
What is your tire pressure?
What is the condition of steering?
When was the last brake check?
Any odd tire wear?
What do you carry in the car every day = total load?

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Technical Quality Auditor.
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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

You could have bought a tank of crap fuel. I'd wait and see...

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'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate

Possibly bad fuel or if you filled up in a place you're not used to, you may not have gotten it full. There's also the possibility that you have a fuel leak that either just started or only manifests itself during highway runs.

Constant light smoke seems normal for that car at speed. Seems like if you were actually burning excessive amount of diesel, you'd see much more smoke - the diesel has to go somewhere.

Try another tankful before you start messing with anything.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

The odometer has gone bad? Very common issue.

The rubber hoses on the bottom of fuel tank are weeping and high speed air blowing past leak points is sucking fuel out.