Ran 2 tanks each with both 91 and 87 just for trial and I get absolutely NO change in fuel mileage. At all. Slightly smoother performance with Premium, but nothing really to warrant the extra cost. I use Sunoco since its the only decently tiered gasoline near my home. However here in college I have access to both Exxon and Shell and switched to them since they are top tier. Either way, octane rating has caused no difference for me at all. Anyone's thoughts on this?