1985 380SE excellent condition - can't even tell the engine's running. Just over 121K (I think...keep reading).

Long story short. I became suspicious of my odometer as a result of very poor average calculations of 12mpg. This was for mostly city driving, accelerating V - E - R - Y slowly (economy gauge out of the red) and using cruise control almost all the time (even on 30mph roads).

I used Microsoft Streets and Trips to measure actual miles covered over the last couple of days. My odometer reported 31.8 miles but I actually travelled 46.7 miles. So the odometer appears to be reporting roughly 68 percent of actual miles travelled.

I went back and re-calculated my mileage figures (I keep a very meticulous log) and if the odometer behavior has been the same over the last month then I actually average 17 to 18 mpg in mixed driving which sounds more consistent with my 380SE experience.

I will of course closely monitor my routes and mileage for the next month to confirm my calculations. Has anyone else experienced this? Any thoughts? What causes this and can it be corrected?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280