What could be the cause of a P0455 large evap leak?

Hello, your "down revs", miles per gallon fuel usage and fuel gauge will not be affected by this evaporative code. A large leak typically would not be an intermittent failure. A loose gas cap is usually a cause. Some kind of other completely detached component can cause this light. If the code returns and remains the same, it will be necessary to continue the smoke testing to find the gross leaks.