What am I doing wrong?

[cbass]

I have the 305/30's on 20x11's with a +51 offset. you should be fine. Mine are damn near square, going to a 275 would look stretched.

 

[Epic510]

Should we all assume your running PREMIUM GAS - your mph is way low like 3 mph low- makes me wander if the car is pulling timing do to bad gas from the pump or the pumpe'. (meaning you/ what the attendant was told to pump) (Jersey thing) and yes a New tune would help- but keep the stock air box for your mod level-

Yep, only run Shell 93. I live in PA so unlike those Jersey boys I pump my own gas. I never had any audible pinging so I don't think it's the fuel.