Run that tank of gas through and get the highest octane rating you can (91 is the highest there?) from a station that you know to have quality gas.
Then dyno in 5th gear.
I recently filled up with nonamebrand 91 octane from a grocery store to use up our 'gas reward points'
Bad idea. My car bogged down off the line and just generally felt unhealthy until I diluted the tank with some 93 octane.