I'm somewhat perplexed by this: with identical driving and traffic conditions and using the same pump at the same station, my mpg can be 28 one time and 24.5 the next. Weather fairly constant. Pattern repeats semi-randomly. 1981 300D.

I thought the odometer must be slipping, but I've clocked numerous 33-mile one-way commutes and all seems appropriate.

What could be causing this? Is it something I should fix on the car?

__________________
Erlend
1981 300D 202K daily driver

MPG will never be constant. Detours, driving faster than usual, passing more frequently and city driving can change the average.

24mpg is still good.

There are a few other variables in calculating fuel economy (I assume that you are measuring miles on your odo vs. gallons it takes to fill a tank.

-Wear on the nozzle - they are not computer-controlled closed-loop systems that automatically adjust for fuel liquid temperature, flow, tank pressure etc... nor do they recieve "gentle so it lasts forever" handling imho.

-position of the nozzle in your car - the nozzle is shut off by a vacuum sensing port on the nozzle - this might trip differently if the nozzle is slightly pushed in or out a bit differently, or if the fuel foams up more (as affected by...

-Temperature outside - colder operation affects power (and fuel volume...), which affects how much farther you push the pedal down....I don't know about you but the thermometer has been impersonating a basketball here in Cincinnati.

I think that you are doing all you can. I figure that an average of 3 mpg calculations is a MINIMUM for measuring fuel economy. You have done well to minimize variables, but I think that there are still too many.

Your readings sound about right for a lightly driven NA 300D. I got the same in my 240D and the turbo guys rarely make it up to 28.

-John

__________________
2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)

The largest variable is the operator when he fills the tank.

The diesel fuel will foam significantly..........and, the only way you can get consistent mpg readings is to fill it right to the top of the filler pipe. This takes another two minutes at the end to allow the foam to settle.

I'm meticulous about filling to the cap and the SD has a perfectly calibrated odometer. It travels the same roads at the same speeds each week. It still varies from 26.5-28.5 mpg on the winter fuel

I'm looking forward to the summer fuel..........the SD climbs to 28.5-30.5 under the exact same driving conditions.

I believe that the cetane of the fuel varies as well, thus affecting economy...my 300D always bumps between 22-24mpg. But when hurricane Rita came through in 05', all my diesels jumped 2-3 mpg in mileage. I'd read that some red fuel and regular sulfur content fuel had been released into the supply to alleviate shortages. After hearing that, a closer examination of the inline filter on my TD revealed traces of red dye...go figgah.

__________________
Never a dull moment at Berry Hill Farm.