Apparently the gas that gives you better power or mileage is higher octane. Engines have knock sensors and the computer ******* ignition timing when knocking occurs. With your tuner, timing probably is kept advanced as much as possible and the results are even more pronounced than stock. Octane has everything to do with it. There are probably other brands that will work as well, but repeated testing would be a pain, and then there is the possibility that additives, ethanol content, summer vs. winter fuel, refinery variations etc. will skew the results. Premium fuel is recommended in the owners manual for some trucks when towing, but time after time owners of such vehicles choose to ignore it.
