I don't think it is simply tires. Unless she is running on 10 psi, I doubt that it could affect mpg that much.

Pindoc, have you got a GPS to check mileage and your speedo and odometer and trip?

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow