I have a feeling that the mileage increases because the tires are inflated to, or past, the specified pressure when the nitrogen is put in, verses under-inflated when filled with air.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine