http://www.elektro.com/~audi/toluene/
If you get an ECU that will let you swap maps on the fly you can make on for city driving on 93 (or 91 if you are so unlucky) and one for the mix. You can then switch maps when you go to the autoX/track/twisty hill and decide to run the mix.
Disclaimer: Bad mix | bad gas = motor rebuild