I got an average of 23 MPG last year running 91 Octane fuel. After installing the Diablo tune My mileage increased from 21 to 23 which made up for the added expense of buying 91 octane over 87.
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2004 Torch Red GT
Flowmaster Am.Thunder cat back, SLP Mid Pipe
Steeda Underdrive pulleys
3.73 gears
Diablosport Predator tune
Grille Delete and Mach 1 spoiler
Rear Wing delete 
Mustang Tuning Shifter
Bullitt Handling Kit
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