90 octane is premium in Alaska, and it sucks as well. I had it tuned for 89 because that is the closest thing to our crappy gas the tuner could find in Dallas. The newer GM ECM's have two different timing curves. When the knock sensors detect detonation, the curve defaults to the less aggressive timing curve. The EMC will not reset to the more aggressive timing curve unless the fuel tank is topped off. It's just better to tune the things for the gas available and turn off the dual curve toggle. I use EFI Live and usually tune my own vehicles, but didn't want to mess with this one because I'd never be able to load it up in third or fourth gear like the tuner can with a chassis dyno.