After you said that, I tried finding out the difference. Seems most people swapping tire widths, even drastically (245 to 275) experienced less than 1mpg difference. It would be hard to even calculate that with consistency.
As for rotors on a GTP:
Front Brake Rotor Diam (in): 10.8
Rear Brake Rotor Diam (in): 10.9
Upgrading to the Impala rotors puts you at 11.9" front.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne