Gasoline:To make it simple, you can always run a HIGHER Octane Fuel, on a LOWER Octane Tune, But not the opposite.Example: If you're tuned for 91 Octane fuel, you may use 93 Octane fuel. If you're tuned for 93 Octane fuel, you may NOT use 91 Octane fuelEthanol FuelsThe above does NOT apply to Ethanol or Ethanol Blends.You can NOT run E30 in an E85 tune, or E85 in an E30 tune.Ethanol Tune Safe Working Ranges:E85 Tunes may use Ethanol Content varying from E60 to E85E30 Tuned should stay as close to E30 as possible +/- 3% (E27-E33)If you are tuned for E30 - You can NOT run 91 or 93 octane on its own. Please follow the content guide above.