It actually depends on the tires. When I had NT05s I have a mpg hit of around 2 or 3.
Car: 2011 Black Mustang v6, 6-Speed Auto, Premium Package
MODS: KW Variant 3 Coilovers w/ Maximum Motorsport CC Plates | J&M Adj. Panhard Rod | AEM Intake | Bama 91R | Borla S-Type Axle-Back | Boss Front Conversion w/ Saleen Grille + Foglight w/ Auto Headlight | Boss Rear Diffuser | Forgestar F14 18x10 +42 w/ Nitto NT05 275/35-18 (all around)| BMR LCA | SVT GT500 BBK|
Streets of Willow(CCW)- 1:29.661
Buttonwillow(CCW)-Let's not talk about it