I've been driving my 1986 190d 2.5 5spd manual for the last 2 years as a commuter. I drive about 1200 miles a month and I regularly calculate my fuel mileage when I fill up. It has always averaged around 37-39 mpg until recently. On the last two tanks (one B20, one petro) I've only gotten about 33 mpg (about a 10% drop), but haven't changed my driving habits, noticed any loss of power or change in performance. What could be the cause of this?


About 5 weeks ago I had new tires put on, a new vacuum pump, a new crankshaft position sensor and had the speed sensor repaired to fix a non-working tach. I could see how these might create a minor change in mpg, but nothing that would cut my efficiency by 10%. Where do I start?

winter diesel sucks don't it.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

oldsinner's probably right; ask if your fuel source has gone to winter blend already. You didn't go to a different tire size, did you? Maybe the pump you used last was a little more sensitive and shut off a bit early, or you squeezed a bit more in with your most recent fillup (?). See what your next tank nets you, but I'll bet it's just the winter blend.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

since you've watched your fuel consumption for 2 years, I trust that you are accustomed to the seasonal fluctuations.

try changing stations for a few fill ups.

tires have a great effect on highway efficiency.

New tires (although the same size) do have a slightly larger radius. Did you change tire sizes?

Currently, my odometer and wrong tire size, provide me with a 6% difference b/w actual miles and displayed miles. I would certainly suggest that fixing the odometer and changing the tires could cause this discrepancy.

take care of any regular maintenance items that you may have over-looked: brakes dragging, alignment, lubricating fluids in drivetrain.. etc.

Get a hold of a friends GPS and confirm your mileage, or take a long trip and use the highway milemarkers to make an educated estimate of your actual discrepancy.

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I normally run Shell diesel but recently visited Murphy's for @ 2 fillups and seen a drop in my fuel economy with my 350sd. Went back to Shell.

You had new tires put on, but make sure the tire pressure is where you usually keep it. Just a few psi too can cause a big drop in milage.

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The tire size did not change, from a bald 195 60 15 to a fresh 195 60 15. I had previously set tire pressure at about 40 psi, while the guy at the tire shop set the pressure on the new tires at 29psi per the sticker on the fuel door.

I live in NC where we haven't yet had our first freeze and most of the leaves are still on the trees. Do they use winter diesel here in Nov.? My mileage was equally poor on the tank of b20 as on the tank of petro, and I've never seen mileage below 35mpg in this car, winter or not.

Do I need more air in the tires? Is there a brand name diesel I can trust?

If you got the higher mpg with 40 psi, and now only have 29 psi . . . that is the biggest contributing factor.

Were the old tires only bald (or more worn out) in the center of the treads?

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82 240D Manual 277K and still rolling!
02 Volvo S60 AWD For Sale

Raising tire pressure up as high as you can stand or the tire allows is one of the key things the hypermiler people do to maximize fuel mileage. Upping yours to 40 will make a difference.

Run a few tanks at 40psi, and report back to us

I've noticed my mpg has dropped too...but only about 2 to 3 mpg. Usually I get 33-34 and for the last two tanks it was 30-32. I use BP diesel. I just figured it is the winter blend diesel that is to blame.

Were you in Chapel Hill last week? I saw a white 190d that sounds similar to yours.

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

OT - Did the tach get fixed? Was the sensor the only problem with it?

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

B20 has less BTUs than D2. You will notice a drop in mileage using B20 over D2 but 10% is too much. It could be coupled with the first winter blend arriving and a low fill up. If it were me I would just continue to check my mileage over the next 2 or 3 fills and see if it 'levels out'. The best way to check fuel mileage is over a few fills as you may put a bit more in one day and a bit less in the next even when the pump stops pumping.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

bmears.....I agree with what has been said regarding tire pressure.
I would start with upping the pressure on the new tires from the 29psi that the shop put them at. You got two things working against you......low tire pressure and a new deep soft tread causing alot more rolling resistance than your old set that was over-inflated with hardly any tread left.

Pick a pressure between the maximum on the side wall of the tire and what is suggested on the spec. plate of the vehicle and see what you get with handling and economy.

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

Did you get the mpg drop figured out?

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

My mpg has dropped down 2 mpg. 25 down to 23 for 4 consecutive fuel ups.

Most fueling stations have switched to winter diesel up here. I'm not too pleased about that.

Oh well

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-Typos courtesy of my mobile phone.