If this is accurate, your doing very well. I also drive about 80/20 highway/city. Best I can get consistently is around 620 miles before I fill er up. Even long trips to Chicago and Knoxville from Northern Virginia don't get me even close to your kind of range. I've tracked mileage for every tank for the past 5 years and throw the receipts in a bag. Next week I'll make my kid set up a spreadsheet and calculate average, best, etc, and post results. Are you doing anything special? Tire pressure? Driving downhill both ways? Believe me, I'd like to know how to bump mpg by 20%. Respectfully dubious.

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900