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There's no meaningful way to measure fuel economy by "km's per tankful" - you have to fill to the brim, note odometer reading, then fill to the brim a few hundred km/mi. later and do the math. It's easy to set up a simple spreadsheet to track these fillups and interesting to see the effect of driving conditions on economy.
My daughter's '83 240 with M46/OD gets 24MPG(US) in mixed city/highway driving, but will turn in about 28MPG with 70-75MPH all-highway use. Those convert to 9.8 and 8.4L/100km, respectively. My son's 81 with M46/OD gives similar numbers.(Take MPG(US) and divide into 235.4 to get L/100km).
As for manual v. automatic, the fuel economy penalty is most significant when the automatic is used in stop/go driving. The later AW boxes feature a torque converter clutch which makes them virtually as efficient at highway speeds as the manuals, even though they will run at a bit higher RPM for a given speed due to their rear axle ratios. My "new" non-turbo 940 automatic returned 30MPG on its maiden highway run, at about 2800RPM. This is 36MPG (Imperial) or 7.8L/100km. At its smog test, the technician said it was burning as cleanly as anything he'd seen in a while, so the engine is in good shape, and tuned correctly.
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Bob (81-244GL B21F, 83-244DL B23F, 94-940Sedan B230F)
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