You should be getting ~20mpg +-2/3 depending on your foot. Best I have ever seen in my 300TE is 22mpg (avg 17), my 260E 24mpg (avg 21) and that was on long runs 60mph for 200mi+.

You should definitely be getting at LEAST 260mi to a tank before the light comes on. 300mi before the light comes on is ~21mpg as that is what I get day-to-day in the 2.6.

Seeing as the O2 sensor is saying the mixture is correct, with the conversion was the correct ECU Module used? Could be a computer error causing the problem.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k