I have an Excel file similar to yours, with almost every tank for my 300D since Jan 2005. My average, which has included a wide range of driving from summer highway to awful winter city, has been 26.25369 with a standard dev of 2.037264. My highest tank was 30.6, although I was sure I had one higher. I've really never done an all-highway tank on this car, which is kind of sad. Haven't done a lot of highway at all in the last couple years, so my mileage has overall gone down ... but last time I did a tank that was half highway/half city, it was 30 mpg.
I haven't added to the sheet for ages ... stops at Jan 2009 ... but I have all the receipts with mileage written on them so I can enter them at some point and catch up.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)