The 11-14 AC systems are Huge pieces of crap, I've replaced the compressor hub 3 times, then it failed and snapped the belt, I have a replacement but Florida has been 100 degrees and I'm not working on it til winter..
It looks like 275/40/18 is the widest 18" that the Indy 500 come in, sorry..
https://www.tirerack.com/tires/tires.jsp?tireMake=Firestone&tireModel=Firehawk+Indy+500&sidewall=Blackwall&partnum=74WR8FHI5&tab=Specs
Firestone Firehawk Indy 500. Just bought a set for my Cobra, fricken excellent tire, lots of other members here agree plus the price is very fair compared to Michelin/Bridgestone.
your not gonna see any significant RWHP gain with your mods from a tune. I'd pick up a used Diablo Predator, load the 93 tune, increase spark advance to 26-28 at WOT over 4K. Throw it on a dyno to see the A/F ratio and adjust fuel with the Predator accordingly. This will be within 10 HP of a...